The following outline is provided as an overview of and topical guide to concepts related to infectious diseases in humans.
Infection – Subsequent to their transmission via environment and evasion or overcoming of defense, the entry/invasion, establishment, and replication of disease-causing microscopic organisms (pathogens or agents) inside a host organism, and the reaction of host tissues to them and to the toxins they produce.
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.
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. 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. 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. 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. Pathogenesis – process by which a disease or disorder develops, including factors which contribute to its onset, progression and maintenance.
Pathogens and related disciplines There are mainly seven types of pathogenic microorganisms: viruses, bacteria, fungi, protozoa, helminths, other parasites and abnormal proteins called prions. Bacteria – microscopic single-celled organisms lacking a nucleus, some of which can cause infections in humans. 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. Viral disease – diseases caused by viruses. Virology – study of viruses, their structure, replication, and role in viral diseases. Subviral agent – smaller than viruses and have only some of their properties. Viroid – small single-stranded, circular RNAs that are infectious pathogens. Satellite virus – subviral agent that depends on the coinfection of a host cell with a helper virus for its replication. Prions – infectious agents composed solely of misfolded proteins, causing neurodegenerative disorders. 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. 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.
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. Protozoan infection 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. List of insect-borne diseases Medical entomology – study of insects and arthropods that impact human health by acting as vectors for infectious diseases.
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.
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. Endogenization – A pathogen becoming fixed into a host's germline, enabling vertical transmission. 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. Immunopathology – study of how a host's immune response can sometimes cause disease or tissue damage. Cytokine storm – pathological reaction in humans and other animals in which the innate immune system causes an uncontrolled and excessive release of pro-inflammatory signaling molecules called cytokines. 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.