Infectious disease

Infectious diseases are illnesses caused by organic pathogens. These pathogens primarily include bacteria and viruses, but also extend to fungi and parasites. Disease-causing parasites include two classes of organisms: protozoa and helminths (worms).

People can contract infectious diseases in multiple ways, including through environmental exposure to pathogenic agents and the consumption of contaminated food or water. Animals and insects can also transmit infectious agents to human hosts, who can spread some infectious diseases to one another. Organic pathogens can cause diseases of varying levels of severity, from mild illnesses to life-threatening syndromes.

Many medical conditions qualify as infectious diseases. Some are relatively common, while others are rare. Public health officials sometimes refer to the “big three infectious diseases” (BTIDs) when discussing infectious diseases as a global issue. The BTIDs are tuberculosis, malaria, and HIV/AIDS. Other infectious diseases such as coronavirus disease 2019 (COVID-19), cholera, dengue fever, influenza, and measles remain of high priority on the global public health circuit. Infectious diseases also cover many other conditions, including a diverse group of illnesses collectively known as neglected tropical diseases (NTDs).

Avian flu (bird flu) is another infectious disease that drew increased attention from the international medical community and caused the death of millions of egg-laying chickens in the United States (US) in the mid-2020s, resulting in egg shortages in the US and Europe. While the disease is rare in humans and it does not appear to transmit easily from person to person, avian flu has a very high fatality rate in humans. Epidemiology and public health experts are working towards the development of therapeutics and other prevention methods for avian flu outbreaks. They monitor the disease closely for potential signs of viral mutations that could facilitate easier human-to-human transmission and potentially cause a pandemic.

Background

Microorganisms, which primarily consist of bacteria but also include other microbes, are constantly present in the human body. They tend to concentrate in several areas of the body, such as the skin, nasal region, and oral cavity. Such microbes are most prevalent in the intestinal tract, where they perform multiple activities with human health benefits. These include producing important nutrients, facilitating digestive processes, supporting the enteric nervous system, and enabling angiogenesis (the growth and development of new blood vessels). People first acquire these microorganisms during birth and early childhood, after which they gradually reach a balanced state within the human body and remain present throughout a person’s life. The collective presence of these microorganisms is known as the human microbiome.

The organisms that belong to the typical human microbiome generally have either neutral or beneficial effects on human health. Infectious diseases generally result from exposure to microorganisms that are not normally a part of the human microbiome. While not all such exposures lead to symptomatic disease, many types of bacteria, viruses, fungi, and parasites can cause illnesses.

Medical experts identify bacteria and viruses as the leading causes of infectious disease, while fungal and parasitic infections are less common but still problematic. Humans can acquire these pathogens in many ways. Some pathogenic agents require close, direct contact to infect a human host. Examples of such contact include consuming contaminated food, water, or animal products; bodily exposure to human or animal waste materials containing infectious agents; and the interpersonal exchange of bodily fluids. Often, people get infectious agents on their hands and introduce them to the internal body by touching their eyes, nose, mouth, or ears, or by failing to sanitize their hands properly before handling or consuming food. Other pathogens can pass from person to person through the air; this is known as airborne transmission.

Multiple factors impact the transmissibility of infectious diseases. Pathogens capable of airborne transmission have a greater potential to spread on a mass scale than pathogens that require close interpersonal contact to infect a new host. The length of time that a given infectious agent can survive in the ambient environment also has a strong impact on its potential to cause large-scale illness in humans. Some such agents can only survive for seconds outside a host, while others have survival times of minutes, hours, days, or longer. Public health experts generally consider the latter class of infectious agents to pose far greater human health risks.

Overview

Scientists and medical professionals use various methods to classify infectious diseases into specific subgroups. Some approaches categorize infectious diseases according to their causative agents, which include bacteria, viruses, fungi, and parasites. Bacteria are single-celled microorganisms. There are millions of types of bacteria. Most are harmless to humans, but some are pathogenic (disease-causing). Viruses are microbes consisting of a protein layer that encases a segment of genetic material. They can only replicate inside a host organism’s body, and their presence within a host frequently causes illness. Fungi are eukaryotic organisms that reproduce through spores. Parasites are organisms that survive by exploiting resources from a host organism’s body, causing illness and other health deficiencies.

Another model groups infectious diseases into three categories: those with high mortality rates, those that cause high rates of long-term or chronic disability, and those with the potential to cause a serious global threat due to their ability to spread quickly and unpredictably. The World Health Organization (WHO) uses this model to evaluate and monitor the relative level of threat posed by a particular infectious disease or a related family of infectious conditions.

One of the most often used models classifies infectious diseases according to their global prevalence and the amount of public health resources used to manage and control their spread. This model has yielded classifications, including the BTIDs and NTDs.

BTIDs:

The BTIDs—tuberculosis, malaria, and HIV/AIDS—are widely recognized as major global infectious diseases. They also consistently rank among the world’s leading infectious diseases as measured by case rates and death tolls.

Tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. These bacteria typically cluster in an infected person’s lungs, though other bodily systems, including the brain, kidneys, and spine, can also become infection sites. The bacterium Mycobacterium tuberculosis can cause latent infections that produce no symptoms and are not transmissible from person to person. However, many cases lead to illness-causing symptoms, including a severe and persistent cough that may produce sputum or blood, along with chest pain, fever, chills, night sweats, loss of appetite, weight loss, fatigue, and weakness.

Malaria is a parasitic disease that typically spreads to humans through infected mosquitoes or, less frequently, zoonotic (animal-to-human) vectors. It does not usually pass from person to person, though rare non-mosquito transmission can occur. Initial signs of malaria include fever and flu-like symptoms such as a headache, muscle aches, fatigue, and chills. As the disease progresses, it can lead to additional symptoms, including nausea, vomiting, diarrhea, jaundice, and anemia. Without treatment, malaria can lead to delirium (extreme confusion), seizures, and kidney failure, which may lead to a coma or death.

Human immunodeficiency virus (HIV) can cause acquired immune deficiency syndrome (AIDS), which is marked by immune system impairments that severely impede the body’s natural ability to fight off infections and diseases. HIV primarily spreads through sexual contact but can also be transmitted through contact with contaminated blood, by sharing needles when engaging in intravenous drug use, and from mothers to children through pregnancy, the process of childbirth, or breastfeeding. People can have latent and asymptomatic HIV infections for many years, which may result in their unwitting transmission of the virus to others. HIV progresses to AIDS when a carrier’s CD4 T-cell (white blood cell) count falls to critically low levels, or when HIV’s immunosuppressive effects lead to a life-threatening secondary condition.

NTDs:

NTDs are a group of more than twenty highly varied medical conditions unified by their complex lifecycle profiles, environmental causes, persistent animal-based reservoirs, and vector-based transmissibility. Experts refer to them as “neglected” because they have historically received less attention and funding than many other diseases. NTDs are caused by all classes of infectious pathogens, including bacteria, viruses, fungi, and parasites, and some are also associated with or exacerbated by exposure to environmental toxins. They are strongly concentrated in tropical regions and are particularly prevalent in low-income tropical areas.

In its complete list of tropical diseases, the WHO includes, in alphabetical order, Buruli ulcer, Chagas disease, dengue and chikungunya, dracunculiasis (Guinea worm disease), echinococcosis, foodborne trematodiases, Hansen’s disease (leprosy), human African trypanosomiasis (sleeping sickness), leishmaniasis, lymphatic filariasis, mycetoma, chromoblastomycosis and other deep mycoses, noma, onchocerciasis (river blindness), podoconiosis, rabies, scabies and other ectoparasitoses, schistosomiasis, soil-transmitted helminthiases, snakebite envenoming, taeniasis/cysticercosis, trachoma, and yaws.

Other Infectious Diseases

Many other transmissible conditions with potentially serious health impacts, including both familiar and rare illnesses, are also considered infectious diseases. Well-known examples include COVID-19; hepatitis A, B, and C; influenza; measles; meningitis; poliomyelitis (polio); salmonella; and varicella (chickenpox). Lesser-known and rare examples include amoebic and bacillary dysentery; diphtheria; hand, foot, and mouth disease; hantavirus; melioidosis; nipah virus disease; scarlatina (scarlet fever); and scrub typhus, among others.

Some less-common infectious diseases have followed outbreaks deemed by public health officials to pose the threat of an epidemic or a pandemic. Examples of these diseases include mpox, West Nile fever, and Zika. A global outbreak of mpox began in 2022 while the COVID-19 pandemic was active, causing concern over the potential for a concurrent global infectious disease crisis. West Nile fever, caused by the mosquito-borne West Nile virus, has resulted in sporadic outbreaks of disease throughout the world in the twenty-first century and is widely considered an emerging international public health threat. Mosquitoes spread the Zika virus, which infects humans with the disease and can cause outbreaks. Periodic increases in Zika virus cases have occasionally raised concerns on a global scale.

Topic Today

BTIDs:

According to WHO estimates, approximately 10.7 million people fell ill with tuberculosis globally in 2024. A large majority occurred in what the WHO refers to as “high-TB burden countries,” which are primarily located in Southeast Asia and Africa. Though tuberculosis is both preventable and treatable, it continued to cause a major mortality toll in 2024, with about 1.23 million deaths worldwide. In 2026, the WHO also issued new recommendations for tuberculosis diagnosis that included near-point-of-care molecular tests, tongue swab samples, and sputum pooling strategies.

Like tuberculosis, malaria is both preventable and treatable. Nevertheless, an estimated 263 million new cases of malaria occurred globally in 2023, resulting in an estimated 597,000 deaths. According to the WHO, approximately 95 percent of reported deaths occurred in the African Region, where many people at risk still lacked access to services needed to prevent, detect, and treat the disease. WHO also reported in 2026 that twenty-five countries in Africa were implementing malaria vaccines through childhood immunization programmes or national malaria control plans.

While the prognosis for people with HIV/AIDS has improved significantly since the mid-1990s, infection rates are highest in low-income regions where many people have limited access to the latest treatments and medications, largely due to economic constraints. According to the Joint United Nations Programme on HIV/AIDS (UNAIDS), approximately 40.8 million people around the world were living with HIV in 2024. UNAIDS also estimated that 1.3 million new HIV infections occurred in 2024, along with 630,000 AIDS-related deaths.

NTDs:

Statistics about NTDs are difficult to generate because the global public health system does not give them much attention. According to estimates from the WHO, NTDs directly affect more than one billion people worldwide, the majority of whom reside in the most underdeveloped areas.

Experts also know that NTDs are most prevalent in non-urban areas, including remote rural regions that are difficult to access. Some NTDs are also strongly concentrated in zones with ongoing political strife and violent conflicts. The United Nations (UN) and the WHO are working to create incentives for the global public health community to treat NTDs with greater urgency. WHO reported that fifty-eight countries had eliminated at least one neglected tropical disease by the end of 2025. WHO also reported in 2025 that official development assistance for neglected tropical diseases had decreased by 41 percent between 2018 and 2023, creating new risks for control and elimination efforts.

Emerging Concerns

While avian flu primarily affects animal populations, it can spread to humans through zoonotic vectors. Person-to-person transmission has been documented, but is very rare. However, epidemiologists and public health experts warn against complacency. Ongoing viral mutations could increase the pathogen’s interpersonal transmissibility profile, and WHO and other public health organizations have documented a trend of emerging viral variants and troubling outbreaks since 2020. In a 2025 joint assessment, the WHO, Food and Agriculture Organization (FAO), and World Organisation for Animal Health (WOAH) stated that the global public-health risk from influenza A(H5) viruses was low, while the risk for occupationally exposed groups ranged from low to moderate. The WHO documented an avian flu human case fatality rate of 54 percent during the twenty-one-year period spanning 2003–2024. Although public-health risk assessments remain low, in a worst-case scenario, a pandemic-scale global outbreak of a highly transmissible avian influenza variant could pose an extremely serious threat to human civilization.

Infectious pathogens have been used in experimental military programs involving biological weapons (bioweapons). Bioweapons include an infectious or toxic agent capable of killing people, animals, and/or plants or causing widespread environmental contamination. Paired with a delivery mechanism, infectious pathogens have numerous potentially catastrophic applications in the military and during warfare. In addition to fears over if and how such weapons could be used in a conventional international military conflict, experts also note the potential for their deployment by terrorist organizations and other rogue actors.

Gain-of-function (GoF) research is one part of a broader debate about dual-use research and the responsible use of the life sciences that drew intense attention during the COVID-19 pandemic. GoF research involves deliberate alterations to viruses to increase their transmissibility, which scientists perform in carefully controlled laboratory settings to study how viruses could evolve in nature and help them prepare preemptive strategies for managing disease outbreaks if they were to occur. Gain-of-function research poses complex ethical questions and debates that began many years before the COVID-19 pandemic, as both its benefits and drawbacks are considerable. Given the devastating consequences of the COVID-19 pandemic, some experts have called for a global moratorium on such research, while others believe the pandemic strengthens the case for continuing or even accelerating this research.

In 2025, the World Health Assembly adopted the WHO Pandemic Agreement, a legally binding international instrument intended to strengthen pandemic prevention, preparedness, and response.

Impact

Infectious diseases have impacted human civilization from its earliest beginnings and, in many cases, have shaped the developmental course of human history. During the early twenty-first century, scientists uncovered evidence that the plagues referenced in both the Old and New Testaments of the Christian Bible appear to be rooted in historical fact; experts now believe that these events were caused by an interconnected series of natural disasters including volcanic eruptions that had major climate impacts and led to the development and spread of new environmental pathogens. During the late stages of the High Middle Ages (ca. 1000–ca. 1350), a bubonic plague pandemic commonly known as the Black Plague or the Black Death engulfed western Eurasia and northern Africa. Experts now estimate that the Black Plague may have caused as many as 200 million deaths, including 40–60 percent of Europe’s entire population. More recent examples include the 1918 influenza pandemic, which killed an estimated 50 million people and contributed to the cessation of World War I (1914–1918), and the COVID-19 pandemic of the early 2020s.

As the most recent major global public health emergency, the COVID-19 pandemic has drawn significant attention from scholars and academics across fields ranging from public health and epidemiology to economics and political science. From the onset of the global pandemic in March 2020 through March 2025, COVID-19 claimed approximately 7.1 million lives worldwide. It also profoundly affected economies, causing what experts have described as one of the most acute global financial crises of all time. COVID-19 generally had the effect of accelerating wealth inequality within and between countries and regions. It also forced unprecedented responses from governments and policymakers, which brought about dramatic disruptions in global trade and productivity. Economists have also associated the emergency economic measures adopted in many countries with high rates of consumer price inflation, which had the short-term effect of eroding consumers’ purchasing power to considerable degrees. Many countries also experienced variable and contradictory public responses to emergency health measures, which frequently had the effect of widening and accelerating internal political divisions that had, in many localities, already been trending toward increasing levels of polarization.

While the infectious disease burden is global in nature and affects countries across the entire spectrum of economic development, low-income and impoverished populations shoulder disproportionately high impacts. Researchers have documented strong, complex, and difficult-to-control links between poverty and infectious diseases, with multiple such illnesses, including the BTIDs and NTDs, sometimes being classified as “infectious diseases of poverty” (IDoPs). IDoPs primarily impact people living in low-income tropical and subtropical regions of the globe in localities such as sub-Saharan Africa, Asia, and Latin America. Experts note that IDoPs exacerbate cycles of poverty that are extremely difficult for impacted populations to escape; those most affected lack the financial and physical resources to seek treatment and stage recoveries, and many have limited or no access to preventative medications or integrated professional care. Therefore, they are often left to manage the ongoing health impacts of these diseases with little outside help, which causes long-term disability and reduced economic productivity that pose ongoing impediments to community-wide economic development.

Young children, particularly those living in impoverished households and regions, are especially susceptible to the health risks posed by the global infectious disease burden. Along with BTIDs, young children in low-income countries are highly vulnerable to infectious conditions including pneumonia and diarrhea caused by foodborne and waterborne illnesses. According to the United Nations Children’s Fund (UNICEF), diarrhea, malaria, and pneumonia combined caused approximately 30 percent of all deaths among children under the age of five in 2019. Observers consider the problem particularly vexatious since medications, vaccinations, and education would make most of these cases completely preventable if they were made more available to vulnerable populations.

Experts anticipate that the global burden of infectious disease will continue to disproportionately impact the world’s most impoverished populations. However, researchers also believe that factors including climate change and rising rates of global socioeconomic integration pose an increasing likelihood of bringing higher rates of infectious diseases to middle- and high-income countries. Public health officials consider climate change a particularly urgent issue. Rising global temperatures are causing longer and more productive breeding seasons for disease-spreading animals and insects. Climate change is also profoundly affecting both marine and freshwater environments, with many models predicting increased future competition over limited water resources that could force people to use unsafe water supplies at greater rates while also potentially sparking violent conflicts.

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