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Outline of infectious disease concepts

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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[edit]

Agent[edit]

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[edit]

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]
    • Bacterial diseases – diseases caused by bacteria.
    • Bacteriology – study of bacteria, their characteristics, growth, and role in infectious diseases.
  • 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
    • 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
    • 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[edit]

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]

Environment[edit]

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.

Transmission[edit]

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[edit]

  • 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[edit]

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[edit]

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[edit]

Exogenous transmission – the transfer and spread of infectious agents between different individuals or from an external source to a host organism.

Cross-species[edit]

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[edit]

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[edit]
Routes[edit]

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
  • Prenatal
Cervico-vaginal
  • Perinatal
Other

Modelling[edit]

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.

Occurrence in population[edit]

  • 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[edit]

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 tract
    • Intestinal – intestinal disease that involves intestinal infection that has material basis in viruses, bacteria, fungi and parasites
  • 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 bones
  • Joint – arthritis that involves infection by a pathogen located in joint syaptic
  • 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 infection
  • Eye

Prevention and control measures[edit]

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[edit]

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[edit]

Non-pharmaceutical intervention – strategies and measures implemented to stop the spread of infection without relying on medication or pharmaceutical products.

Emerging infectious disease[edit]

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]

Miscellaneous concepts[edit]

History of infectious diseases[edit]

Individual diseases[edit]

People[edit]

  • Alexander Fleming – Scottish physician and biologist (1881–1955)
  • Maurice Hilleman – American vaccinologist (1919–2005)
  • Edward Jenner – English physician and pioneer of vaccines (1749–1823)
  • Robert Koch – German physician and bacteriologist (1843–1910)
  • Louis Pasteur – French chemist and microbiologist (1822-1895)
  • Jonas Salk – American inventor of the polio vaccine (1914–1995)
  • Ignaz Semmelweis – Early pioneer of antiseptic procedures
  • John Snow – English epidemiologist and physician (1813–1858)
  • Israel Jacob Kligler – microbiologist (1888-1944)

Major epidemics and pandemics[edit]

Other[edit]

References[edit]

  1. ^ a b c d e f g h i j k l m n o p q r s t u v "Glossary". What you need to know about Infectious Disease. National Academy of Sciences. Retrieved June 7, 2024.
  2. ^ a b "Comprehensive Glossary for Infectious Disease Modelling". National Collaborating Centre for Infectious Diseases. Retrieved June 8, 2024.
  3. ^ "Types of Microbes". What you need to know about Infectious Disease. National Academy of Sciences. Retrieved June 7, 2024.
  4. ^ a b c d e f "Glossary". National Infection Prevention and Control Manual (UK). Retrieved June 7, 2024.
  5. ^ Institute of Medicine (US) Forum on Emerging Infections (2001), "APPENDIX A, Glossary and Acronyms", in Davis JR; Lederberg J (eds.), Emerging Infectious Diseases from the Global to the Local Perspective: A Summary of a Workshop of the Forum on Emerging Infections, Washington (DC): National Academies Press (US)

External links[edit]

Wikimedia Commons has media related to Infectious diseases and disorders.
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