The following outline is provided as an overview of and topical guide to concepts related to infectious diseases in humans.
Infection – transmission, entry/invasion after evading/overcoming defense, establishment, and replication of disease-causing microscopic organisms (pathogens) inside a host organism, and the reaction of host tissues to them and to the toxins they produce.[1]
Infectious disease – illness or disorder when pathogenic (disease-causing) microorganisms, such as bacteria, viruses, fungi, or parasites invade and multiply within the body of a host organism and release toxins, causing various clinical symptoms which can potentially lead to severe health complications or even death. Infectious diseases can be spread, directly or indirectly, from person to person, from animal to animal, or from animals to humans (zoonotic diseases), or through environmental exposure. They can be treated with medical interventions or prevented before they happen.[1]
Determinants
Agent
Infectious agent (also known as pathogen) – a biological entity (microscopic or macroscopic) that causes disease or illness in its host by invading and multiplying within the host's body, disrupting its normal physiological functions and leading to clinical symptoms (i.e. an infectious disease).
- Biofilm - sticky microbial communities adhering to surfaces, facilitating bacterial survival and resistance to treatments.
- Germ theory of disease - paradigm that microorganisms are the principal causative agents of infectious diseases.[1]
- Infectivity - ability of a pathogen to establish itself and cause infection in a susceptible host
- Infectious dose - quantity of pathogen required to cause infection in a host organism.
- Pathogenicity - ability of a pathogenic microorganism to cause disease in a host organism.
- Attack rate – proportion of a population of susceptible individuals who get infected and develop illness during an outbreak over a certain period of time.[2]
- Quorum sensing - Cell-to-cell communication system used by some pathogenic bacteria, potentially triggering virulence factor production.
- Virulence - degree of pathogenicity, measured by the severity of an infectious disease based on damage caused and the host's immune response.
- Endotoxin - toxin released from the outer membrane of certain gram-negative bacteria, triggering inflammation.
- Exotoxin - toxin secreted by bacteria into the surrounding environment (damaging host tissue)
- Case fatality rate - proportion (percentage) of individuals diagnosed with a specific infectious disease who die from it.
- Virulence factors - molecule or characteristic possessed or structure produced by pathogens that increase their ability to cause disease in a host organism.
- Antimicrobial resistance (including antibiotic resistance) - ability of microorganisms to resist the effects of medications designed to kill or inactivate them.[1]
- Drug resistance - reduction in effectiveness of a medication such as an antimicrobial or an antineoplastic in treating a disease.
- Horizontal gene transfer - Transfer of genetic material between different organisms of the same generation, allowing them to acquire new traits, including resistance to antibiotics.
- Multidrug-resistant bacteria/Superbug - bacteria resistant to multiple classes of antibiotics, with limited treatment options.
- Host tropism - preference of a pathogen for a particular host species, cell type, or tissue.
Pathogens and related disciplines
There are mainly seven types of pathogenic microorganisms: viruses, bacteria, fungi, protozoa, helminths, other parasites and abnormal proteins called prions.[3]
- Bacteria – microscopic single-celled organisms lacking a nucleus, some of which can cause infections in humans.[1]
- Viruses - microscopic pathogens consisting of genetic material surrounded by a protein coat, requiring living cells of host organisms to replicate.[1]
- Viral disease – diseases caused by viruses.
- Virology – study of viruses, their structure, replication, and role in viral diseases.
- Prions – infectious agents composed solely of misfolded proteins, causing neurodegenerative disorders.[1]
- Prion diseases – progressive, incurable, and fatal neurodegenerative diseases associated with prions, affecting humans and animals.
- Fungi – eukaryotic spore-forming organisms distinct from plants, animals, and bacteria including single-cell yeasts and multi-cell molds, some of which can cause infections in humans.[1]
- Fungal infection/Mycosis/Fungal disease – infection (and infectious disease) caused by fungi.
- Mycology – study of fungi, their growth, and their role in fungal infections.
- Parasite - organism benefitting at the cost of a host organism.[1]
- Parasitic disease – infectious disease caused or transmitted by a parasitePages displaying wikidata descriptions as a fallback
- Parasitology – study of parasites, their life cycles, and the diseases they cause in humans and animals.
- Protozoa – single-celled eukaryotic organisms that can live as parasites within a host.[1]
- Helminths – macroscopic parasitic worms that can live in different organs and tissues of their host
- Helminthiasis/Worm infection – - parasitic worm infections caused by various helminths (i.e. roundworms, tapeworms, and flukes) residing in different organs and tissues, causing discomfort, nutritional deficiencies, and potentially organ damage.
- Infestation - overgrowth of parasites (like lice, mites, or worms) on or within a host, often causing irritation, discomfort, and potential secondary infections due to scratching or burrowing.
- Insect vectors - usually an arthropod such as flea, tick or mosquito carrying a pathogen from one host to another.[1]
Host
Host – a living organism, typically a human or animal, that harbors a pathogenic microorganism, providing the environment and resources necessary for it to survive, grow, and potentially reproduce or multiply. There is a symbiotic relationship between the two organisms, which can be commensal (mutually beneficial), but in the case of infectious diseases, harmful for the host.[1]
- Burn – causing damage to the skin and underlying tissues, compromising the body's primary defense barrier and increasing susceptibility to infections.
- Comorbidity – presence of one or more additional medical conditions that can increase susceptibility to infections.
- Diabetes – Diabetes impairs immune function and causes poor circulation, increasing susceptibility to infections and complicating healing.
- Host–pathogen interaction – host's immune defenses and the pathogen's strategies to invade and survive, determining the outcome and severity of infections
- Immune response – body's defense mechanism to recognize, fight, and eliminate pathogens, which influences the susceptibility, severity, and resolution of infections.
- Immunodeficiency – weakened or compromised immune system that reduces the body's ability to effectively fight off pathogens, leading to increased susceptibility and severity of infections.
- Immunosuppression – deliberate reduction or impairment of the immune system's function due to medical treatments or conditions, causing increased vulnerability to infections.
- Microbiome health – balance and diversity of the microbial communities in the body, influencing the immune response and overall susceptibility to infections.
- Opportunistic infection – infection caused by pathogens that take advantage of a weakened or compromised immune system.
- Risk of infection – Likelihood of contracting an infection, which is influenced by a host's inherent susceptibility (age, immune status, genetics) or external exposures (behavior, environment).
- Susceptible individual – individual lacking immunity and at risk of infection by a specific pathogen.
- Age (Immunosenescence) – age-related decline in the immune system's function, making older adults more susceptible to infections.
- Sex/Gender – susceptibility to infection due to hormonal differences, immunological variations and differences in behavioral patterns between males and females.
- Nutrition status – nutrient and micronutrient deficiencies increases susceptibility to infections.
- Vaccination status – vaccination can lower the risk of infection by preparing the immune system to recognize and efficiently combat specific pathogens.
- Genetic predisposition – Variations in immune Genes, susceptibility genes and major histocompatibility complex (MHC) genes and genetic predisposition to certain chronic inflammatory disease, contributing to weakening of the immune system.
- Immunogenetics – field investigating the complex interplay between genes and the immune system affecting infection risk.
- Behavioral/lifestyle factors
- Smoking – weakens the immune system and damages lung tissue, making individuals more susceptible to respiratory infections.
- Sleep deprivation – weakens the immune system's ability to fight off pathogens.
- Poor oral hygiene – creates a reservoir for bacteria that can potentially enter the bloodstream and cause infections elsewhere in the body.
- Physical inactivity – weakens immune function, reduces lung capacity, increases risk of chronic diseases and reduces blood flow, making a person more susceptible to infections.
- Substance abuse – Drug and alcohol use can impair the immune system and increase susceptibility to infections.
- Unprotected/unsafe sex – increases the risk of contracting sexually transmitted infections.
- Pregnancy – physiological changes, hormonal changes and immunological changes occurring in the mother's body and not meeting the nutritional demands can lead to infections.
- Stress levels – Chronic stress can suppress the immune system, potentially making the body more vulnerable to infection.
Environment
Environment – the external conditions and factors such as climate, sanitation, human behavior and living conditions that influence the spread, survival, and transmission of pathogens and the risk of infectious diseases.
- Access to water and sanitation – Access to clean, safe water and proper sanitation facilities for waste disposal (toilets, latrines) are crucial to prevent waterborne and fecal-oral transmission of pathogens.
- Air quality – poor air quality with high levels of particulate matter (PM) can irritate the respiratory tract and potentially increase susceptibility to respiratory infections; chronic inflammation caused by exposure to pollutants weaken the immune system.
- Biodiversity loss – disrupts the natural balance that can keep pathogen populations in check, force wildlife into closer contact with humans and domestic animals leading to zoonosis, eliminate reservoir hosts leading to the emergence of new pathogens in the remaining wildlife populations and forcing the pathogen to seek new hosts, including humans.
- Climate change – expands the geographic range of insect vectors, favors the survival and reproduction of some pathogens, disrupts ecosystems, worsens existing health problems, etc. impacting in complex ways and making people more susceptible to infections.
- Climate zones
- Commerce
- Deforestation
- Ecology
- Humidity
- Injection drug use
- Natural disaster
- Poultry and livestock
- Poverty
- Travel
- Urbanization
- Vector control
- War and conflict
Transmission
Transmission – the process by which an infectious agent is spread from one host (an infected person) or environment (a reservoir) to another susceptible host person, facilitating the spread of disease.
Basic concepts
- Asymptomatic carrier – individual harboring a pathogen without showing symptoms (subclinical infection) and still capable of transmitting it to others.
- Chain of infection – sequential steps or process required for an infectious agent to pass from a source host to a susceptible host.
- Fomite – inanimate object contaminated with infectious agents facilitating their transmission between individuals/hosts.
- Host – organism providing shelter and sustenance to a pathogen, helping to survive and potentially replicate to cause health problems.
- Incubation period – time interval between exposure to a pathogen and the onset of clinical symptoms.[1]
- Index case/Patient zero – first identified case in an outbreak investigation, serving as a guide to public health interventions.
- Infectious period – time interval during which an infected individual can transmit the pathogen to other susceptible hosts.
- Latent period – time interval between exposure to a pathogen and when the individual/host becomes infectious, capable of transmitting pathogens.
- Natural reservoir – animal population or environment in which a pathogen normally lives and multiplies and can be transmitted directly or indirectly to humans.
- Opportunistic infection – infection caused by otherwise harmless organisms, taking advantage of a weakened immune system, thus becoming pathogens.
- Silent/Subclinical/Asymptomatic infection – infection with no apparent noticeable signs or symptoms but still capable of spreading, possibly due to a weak immune response or the nature of the pathogen.
- Superinfection – secondary infection occurring on top of a pre-existing one, often with a different pathogen and potentially leading to complex clinical presentations and treatment challenges.
- Super-spreader – infected individual transmitting a pathogen to a disproportionately large number of others (secondary cases) compared to others in an outbreak.
- Viral load – quantity of viral particles in a given volume of (typically per mililiter of) a patient's blood or other body fluids.
- Window period – time interval between exposure to an infection and the reliable detection (by a diagnostic test) of the presence of the pathogen or its specific antibodies in the body.
Modes
Transmission mode – the mechanism through which an infectious agent travels from a source (an infected host or a reservoir) to a new susceptible host.
Endogenous
Endogenous transmission – the spread of infectious agents within a single host organism, typically from their normal habitat in one part of the body to a new location within the body, rather than between individuals.
Exogenous
Exogenous transmission – the transfer and spread of infectious agents between different individuals or from an external source to a host organism.
Cross-species
Cross-species transmission (or host jump) – the transmission of infectious agents from one species (original animal reservoir) to a new one, potentially leading to infections in the recipient species.
- Spillover infection – cross-species transmission of pathogens from a domestic or wildlife animal reservoir to a new human host.
- Vector – organism, typically an insect or arachnid, that transmits pathogens from an infected host to a susceptible host individual.
- Zoonosis – infectious disease transmissible from animals to humans.
- Reverse zoonosis – transmission of pathogens from humans to infect animals.
Human-to-human / Cross-infection
Human-to-human transmission (also known as cross-infection) – direct or indirect transmission of infectious agents from one individual to another, leading to the spread of disease within a population.
- Contagious disease – Infectious disease readily spread by pathogen transmission
- Contagion – disease-causing infectious agent spread by direct or indirect contact.[1]
- Source
- Community-acquired
- Nosocomial/Hospital-acquired infection – An infection occurring in a patient within a hospital or a healthcare in a hospital or other healthcare facility, which was not present or incubating at the time of admission, and often associated with invasive procedures or compromised immune states.[4]
- Iatrogenic/Medical care – Causation of harm by any medical activity
- Generational difference
- Breakthrough infection – Infection following vaccine administration
Environment-to-human
Routes
Routes of transmission – specific pathways through taken by infectious agents to travel from one infected host or reservoir to a new host.
- Respiratory
- Linked to Vascular system (Blood vessels)
- Gastrointestinal (Alimentary canal)
- Cutaneous (Skin)
- Genitourinary
- Trans-placental
- Cervico-vaginal
- Other
Modelling
Mathematical modelling of infectious disease – mathematical equations, tools and simulations to represent the dynamics of disease transmission within a population in order to understand and predict them for disease control and public health decision-making.
- Agent-based model – Simulation of individual agent (human or animal) behavior and interactions within a defined environment to understand disease spread dynamics.
- Animal disease model – Mathematical model used in veterinary epidemiology to understad zoonotic disease spreading dynamics in animal populations and predict zoonotic spillover (animal to human transmission).
- Attack rate – proportion of a population of susceptible individuals who get infected and develop illness during an outbreak over a certain period of time.[2]
- Basic reproduction number – Average expected number of secondary cases infected by a single primary case in a completely susceptible population, useful for assessing epidemic potential.
- Compartmental models in epidemiology – Mathematical models that group individuals of a population into compartments based on infection status (such as susceptible, infected, recovered) to simulate disease dynamics and transmission patterns over time.
- Critical community size – Minimum population size needed for a disease to persist indefinitely within a community without new introductions, influenced by factors like transmission dynamics and immunity.
- Force of infection – Rate at which susceptible individuals acquire infection from infectious individuals, used to model incidence rate (number of infected individuals) and transmission dynamics.
- Herd immunity – Indirect protection of susceptible individuals from infectious diseases when a large proportion of a population becomes immune, either through vaccination or prior infection, reducing overall transmission risk.
- Infection rate – Rate at which new infections occur within susceptible individuals over a defined period.
- Machine learning – techniques enabling computers to analyze large datasets and identify patterns in disease spread, thus learning to forecast and detect disease outbreak and improve response strategies.
- Multiplicity of infection – Number of different pathogen strains (viral particles/genomes) infecting a single host at the same time, influencing disease severity, transmission dynamics, and evolutionary pathways.
- Serial interval – Time between symptom onset in a primary case and symptom onset in a secondary case infected by the primary case, useful for understanding disease spread patterns and assessing control measures.
- WAIFW matrix – (Who acquires infection from Whom, At what rate, For how long, and With what consequences) mathematical framework/matrix to quantify and summarize key transmission parameters for a disease, such as contact rates and probabilities of infection.
Occurrence in population
- Disease cluster – grouping of cases of a disease occurring closely in a specific time and place, suggesting possible local transmission.
- Endemic – constant presence of a disease within a specific geographic area or population at a predictable baseline level.[1]
- Epidemic – sudden, unexpected increase in the number of cases of a disease in a specific population and geographic area.[1]
- Epidemic curve – graphical representation of the rise, peak, and decline of the number of cases of an epidemic disease over time.
- Farr's laws – principles describing the relationship between population density, poverty, and the occurrence of infectious diseases.
- Holoendemic – highly prevalent and constantly present disease occurrence in a population or geographic area, affecting most individuals from an early age.
- Hyperendemic – consistently and persistently high incidence and/or prevalence of disease occurrence within a particular population or geographic area, equally affecting all age groups.
- Incidence – rate of new cases of a disease occurring in a specific population within a defined time period, typically expressed per 1,000 or 100,000 individuals.
- Inequality in disease – disparities in the burden (distribution and impact) of infectious diseases among different population groups, often due to social determinants of health.
- Mesoendemic – moderate level of constant disease prevalence within a specific geographic area or population.
- Disease outbreak – sudden unexpected increase in the occurrence of the cases of a particular disease cases for a given time period and a specific geographical area.[1]
- Pandemic – widespread epidemic, often affecting multiple countries or continents and involving a large number of people.[4]
- Prevalence – proportion of individuals in a population affected by a disease at a specific point in time.
- Seasonality – fluctuation in disease occurrence in different seasons or times of the year, often linked to environmental factors.
- Geographic distribution – spatial pattern of how disease cases are located and spread across different regions.
- Sporadic – scattered, irregular occurrence of disease cases without any recognizable pattern in space and time, often without any common source of infection.
- Syndemic – co-occurrence and interaction of two or more diseases or health conditions in individuals or a population, exacerbating their negative health outcomes.
- Twindemic – simultaneous occurrence of two epidemics, where two infectious diseases concurrently circulate in a population, potentially straining public health resources and increasing healthcare burden.
Anatomical location
The specific area or organ within the body where pathogenic microorganisms invade and proliferate, leading to localized or systemic disease.
- Respiratory
- Gastrointestinal – disease involving the gastrointestinal tractPages displaying wikidata descriptions as a fallback
- Intestinal – intestinal disease that involves intestinal infection that has material basis in viruses, bacteria, fungi and parasitesPages displaying wikidata descriptions as a fallback
- Genitourinary/Urogenital disease - infection of organs involved in the excretion of fluids and reproduction.[1]
- Nervous system
- Skin/Cutaneous – Infections of the skin
- Soft tissue
- Bone – Infection of the bonesPages displaying short descriptions of redirect targets
- Joint – arthritis that involves infection by a pathogen located in joint syapticPages displaying wikidata descriptions as a fallback
- Cardiovascular
- Systemic/Generalized – Condition which affects multiple organs or the whole body
- Blood – Life-threatening response to infection
- Tooth – Infection originating in a tooth
- Mouth
- Fetus – placenta disease that is an inflammation of the fetal membranes (amnion and chorion) due to a bacterial infectionPages displaying wikidata descriptions as a fallback
- Eye
Prevention and control measures
Infection prevention and control – strategies, measures and practices designed and implemented to prevent the spread of infectious diseases within healthcare settings and communities, reducing the risk of infections among patients, healthcare workers, and the general population.
Pharmaceutical
Pharmaceutical prevention and control – Use of medications, vaccines, and other pharmaceutical interventions to prevent the spread of infectious diseases, treat infected individuals, and manage outbreaks within populations.
- Antibiotic - medication used to prevent and treat bacterial infections by killing or inhibiting bacteria.[1]
- prophylactic antibiotic - administration of antibiotics to an uninfected individual to prevent the development of an infection, typically before a surgical procedure or in individuals at high risk of infection
- Antifungal - medication used to treat fungal infections.
- Anthelmintic - medication used to treat parasitic worm infections.
- Ascaricide - medication used to specifically treat roundworms.
- Antimicrobial - substance that kills, reduces or inhibits the growth of microorganisms.
- Antimicrobial stewardship - strategies to ensure appropriate and effective use of antimicrobial agents to prevent microbial resistance.
- Antiseptic - Chemical substance applied to skin or surface to reduce pathogens and prevent infection, but not strong enough to sterilize.
- Antiviral - Medication to treat or prevent viral infections.
- Asepsis - Techniques and practices to prevent contamination in healthcare settings and maintain sterility.
- Combination therapy - use of multiple medications (multiple active ingredients) simultaneously to treat a single disease, enhancing therapeutic efficacy and reducing the risk of drug resistance.
- Pharmacovigilance / Drug safety - science and activities involved in the monitoring, detection, assessment, understanding, and prevention of adverse effects or any other drug-related problems to secure their safety and ensure that potential benefits outweigh the risks.
- Immunization - process of strengthening immune response and increase protection against infectious diseases by receiving vaccines.[1]
- Immunotherapy - treatment boosting the body's immune response against disease.
- Inoculation - introduction of a pathogen or antigen into the body to stimulate immunity.
- Phage therapy - treatment using bacteriophage viruses that infect and kill bacteria to combat bacterial infections.
- Pre-exposure prophylaxis - taking medication taken before potential exposure to prevent infectious disease.
- Post-exposure prophylaxis - taking medication taken after potential exposure to prevent infectious disease.
- Drug repositioning - use of existing medications for new indications or purposes not initially intended, including treating infections.
- Vaccine - biological preparation (usually containing weakened or inactive pathogens) to stimulate the immune system to fight against future infection.[1]
- Vaccination - administration of a vaccine to stimulate the immune system and prevent future infection.
- Vaccine efficacy/effectiveness - measurement of the ability of a vaccine to produce desired outcome (protection of individuals or populations from a specific disease) in controlled conditions/real-world settings.
- Booster dose - additional dose of a vaccine to enhance or prolong immunity or to strengthen waning immunity.
- Vaccine resistance - ability of a pathogen to evade the protective effects of a vaccine, potentially leading to breakthrough infections.
- Vaccine-preventable disease - disease preventable by vaccination.
- Ring vaccination - strategy of targeted vaccination of close contacts of infected individuals to prevent further spread and control outbreaks.
Non-pharmaceutical
Non-pharmaceutical intervention – strategies and measures implemented to stop the spread of infection without relying on medication or pharmaceutical products.
- Contact tracing – Finding and identifying people in contact with someone with an infectious disease
- Cordon sanitaire – Quarantine of a geographic area
- Disease surveillance – Monitoring spread of disease to establish patterns of progression
- Disinfection – Antimicrobial agent that inactivates or destroys microbes
- Flattening the curve – Public health strategy
- Hygiene – Practices performed to preserve health
- Food hygiene – Scientific discipline
- Hand washing – Act of cleaning one's hands
- Gloves – Single-use glove worn during medical examinations and procedures
- Isolation – Measure taken to prevent contagious diseases from being spread
- Stay-at-home order/Lockdown – Quarantine strategy
- Notifiable disease – Disease that must be reported to authorities
- Protective sequestration – Public health term
- Public health – Promoting health through informed choices
- Outbreak response – Measures to reduce the spread of an infectious disease
- Quarantine – Epidemiological intervention to prevent disease transmission
- Respiratory source control – Strategy for reducing disease transmission
- Safe sex – Ways to reduce the risk of acquiring STIs
- Sanitation – Public health conditions related to clean water and proper excreta and sewage disposal
- Screening – Brief medical evaluation to detect unnoticed health problems
- Social distancing – Infection control technique by keeping a distance from each other
- Standard infection control precautions - fundamental infection prevention and control practices to be adopted by staff in healthcare settings, irrespective of patient's infectious status.[4]
- Sterilization - process of destroying infectious agents (as well as all other life forms) from a surface, fluid, or biological medium by using heat, chemicals, irradiation, high pressure, filtration, or a combination of these methods.[1]
- Transmission-based precautions – Infection-control precautions in health care
- Travel restrictions – Restriction of all means of travel
- Universal precautions – Medical standard for contact avoidance
- Vector control – Methods to limit or eradicate the mammals, birds, insects etc. which transmit disease pathogens
- Wastewater surveillance – Monitoring wastewater for contaminants
- Zoning
Emerging infectious disease
Emerging infectious disease – diseases caused by newly identified or reemerging infectious agents that have recently rapidly increased in incidence or geographic range, posing a potentially significant threat to public health in the near future.[5]
