As of March 2020, it was unknown if past infection provides effective and long-term immunity in people who recover from the disease. Immunity is seen as likely, based on the behaviour of other coronaviruses, but cases in which recovery from COVID-19 have been followed by positive tests for coronavirus at a later date have been reported. These cases are believed to be worsening of a lingering infection rather than re-infection.

Adenovirus can cause severe necrotizing pneumonia in which all or part of a lung has increased translucency radiographically, which is called Swyer-James Syndrome. Severe adenovirus pneumonia also may result in bronchiolitis obliterans, a subacute inflammatory process in which the small airways are replaced by scar tissue, resulting in a reduction in lung volume and lung compliance.

The impact of the pandemic and its mortality rate are different for men and women. Mortality is higher in men in studies conducted in China and Italy. The highest risk for men is in their 50s, with the gap between men and women closing only at 90. In China, the death rate was 2.8 percent for men and 1.7 percent for women. The exact reasons for this sex-difference is not known, but genetic and behavioural factors could be a reason. Sex-based immunological differences, lesser prevalence of smoking in women and men developing co-morbid conditions such as hypertension at a younger age than women could have contributed to the higher mortality in men. In Europe, 57% of the infected individuals were men and 72% of those died with COVID-19 were men. As of April 2020, the US government is not tracking sex-related data of COVID-19 infections. Research has shown that viral illnesses like Ebola, HIV, influenza and SARS affect men and women differently. A higher percentage of health workers, particularly nurses, are women, and they have a higher chance of being exposed to the virus. School closures, lockdowns and reduced access to healthcare following the 2019–20 coronavirus pandemic may deferentially affect the genders and possibly exaggerate existing gender disparity.

Safe and effective adenovirus vaccines were developed for adenovirus serotypes 4 and 7, but were available only for preventing ARD among US military recruits, and production stopped in 1996. Strict attention to good infection-control practices is effective for stopping transmission in hospitals of adenovirus-associated disease, such as epidemic keratoconjunctivitis. Maintaining adequate levels of chlorination is necessary for preventing swimming pool-associated outbreaks of adenovirus conjunctivitis.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus strain that causes coronavirus disease 2019 (COVID-19), a respiratory illness. It is colloquially known as the coronavirus, and was previously referred to by its provisional name 2019 novel coronavirus (2019-nCoV). SARS-CoV-2 is a positive-sense single-stranded RNA virus. It is contagious in humans, and the World Health Organization (WHO) has designated the ongoing pandemic of COVID-19 a Public Health Emergency of International Concern. Because the strain was first discovered in Wuhan, China, it is sometimes referred to as "Wuhan virus" or "Wuhan coronavirus". Since the WHO discourages the use of names based on locations such as MERS, and to avoid confusion with the disease SARS, it sometimes refers to SARS-CoV-2 as "the COVID-19 virus" in public health communications. The general public frequently calls both SARS-CoV-2 and the disease it causes "coronavirus", but scientists typically use more precise terminology.

Taxonomically, SARS-CoV-2 is a strain of Severe acute respiratory syndrome-related coronavirus (SARSr-CoV). It is believed to have zoonotic origins and has close genetic similarity to bat coronaviruses, suggesting it emerged from a bat-borne virus. An intermediate animal reservoir such as a pangolin is also thought to be involved in its introduction to humans. The virus shows little genetic diversity, indicating that the spillover event introducing SARS-CoV-2 to humans is likely to have occurred in late 2019.

Epidemiological studies estimate each infection results in 1.4 to 3.9 new ones when no members of the community are immune and no preventive measures taken. The virus is primarily spread between people through close contact and via respiratory droplets produced from coughs or sneezes. It mainly enters human cells by binding to the receptor angiotensin converting enzyme 2 (ACE2).

Human-to-human transmission of SARS-CoV-2 has been confirmed during the 2019–20 coronavirus pandemic. Transmission occurs primarily via respiratory droplets from coughs and sneezes within a range of about 1.8 metres (6 ft). Indirect contact via contaminated surfaces is another possible cause of infection. Preliminary research indicates that the virus may remain viable on plastic and steel for up to three days, but does not survive on cardboard for more than one day or on copper for more than four hours; the virus is inactivated by soap, which destabilises its lipid bilayer. Viral RNA has also been found in stool samples from infected individuals.

The degree to which the virus is infectious during the incubation period is uncertain, but research has indicated that the pharynx reaches peak viral load approximately four days after infection. On 1 February 2020, the World Health Organization (WHO) indicated that "transmission from asymptomatic cases is likely not a major driver of transmission". However, an epidemiological model of the beginning of the outbreak in China suggested that "pre-symptomatic shedding may be typical among documented infections" and that subclinical infections may have been the source of a majority of infections.

There is some evidence of human-to-animal transmission of SARS-CoV-2, including examples in felids. Some institutions have advised those infected with SARS-CoV-2 to restrict contact with animals.

In 2012, the World Health Organization estimated that vaccination prevents 2.5 million deaths each year. If there is 100% immunization, and 100% efficacy of the vaccines, one out of seven deaths among young children could be prevented, mostly in developing countries, making this an important global health issue. Four diseases were responsible for 98% of vaccine-preventable deaths: measles, "Haemophilus influenzae" serotype b, pertussis, and neonatal tetanus.

The Immunization Surveillance, Assessment and Monitoring program of the WHO monitors and assesses the safety and effectiveness of programs and vaccines at reducing illness and deaths from diseases that could be prevented by vaccines.

Vaccine-preventable deaths are usually caused by a failure to obtain the vaccine in a timely manner. This may be due to financial constraints or to lack of access to the vaccine. A vaccine that is generally recommended may be medically inappropriate for a small number of people due to severe allergies or a damaged immune system. In addition, a vaccine against a given disease may not be recommended for general use in a given country, or may be recommended only to certain populations, such as young children or older adults. Every country makes its own vaccination recommendations, based on the diseases that are common in its area and its healthcare priorities. If a vaccine-preventable disease is uncommon in a country, then residents of that country are unlikely to receive a vaccine against it. For example, residents of Canada and the United States do not routinely receive vaccines against yellow fever, which leaves them vulnerable to infection if travelling to areas where risk of yellow fever is highest (endemic or transitional regions).

A "vaccine-preventable disease" is an infectious disease for which an effective preventive vaccine exists. If a person acquires a vaccine-preventable disease and dies from it, the death is considered a vaccine-preventable death.

The most common and serious vaccine-preventable diseases tracked by the World Health Organization (WHO) are: diphtheria, "Haemophilus influenzae" serotype b infection, hepatitis B, measles, meningitis, mumps, pertussis, poliomyelitis, rubella, tetanus, tuberculosis, and yellow fever. The WHO reports licensed vaccines being available to prevent, or contribute to the prevention and control of, 25 vaccine-preventable infections.

The disease is associated with high morbidity and mortality and mainly affects children under the age of twelve in the poorest countries of Africa. Children in Asia and some countries of South America are also affected. Most children who get the disease are between the ages of two and six years old. The WHO estimates that 500,000 people are affected, and that 140,000 new cases are reported each year. The mortality rate is approximately 90 percent.

"Fusobacterium necrophorum" and "Prevotella intermedia" are important bacterial pathogens in this disease process, interacting with one or more other bacterial organisms (such as "Borrelia vincentii, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Staphylococcus aureus", and certain species of nonhemolytic "Streptococcus").

It is often reported as a sequela to acute necrotising ulcerative gingivitis. Predisposing factors include:

- malnutrition (particularly A-and B-vitamins) or dehydration

- poor hygiene, particularly oral

- unsafe drinking water

- proximity to unkempt livestock

- recent illness

- an immunodeficiency disease, including AIDS

- measles

- smoking

Occupational exposure to chemicals, dusts, radiation, and certain industrial processes have been tied to occupational cancer. Exposure to cancer-causing chemicals, also called Carcinogens, may cause mutations that allow cells to grow out of control, causing cancer. Carcinogens in the workplace may include chemicals like anilines, chromates, dinitrotoluenes, arsenic and inorganic arsenic compounds, beryllium and beryllium compounds, cadmium compounds, and nickel compounds. Dusts that can cause cancer leather or wood dusts, asbestos, crystalline forms of silica, coal tar pitch volatiles, coke oven emissions, diesel exhaust and environmental tobacco smoke. sunlight; radon gas; and industrial, medical, or other exposure to ionizing radiation can all cause cancer in the workplace. Industrial processes associated with cancer include aluminum production; iron and steel founding; and underground mining with exposure to uranium or radon.

Other factors that play a role in cancer include:

- Personal characteristics such as age, sex, and race

- Family history of cancer

- Diet and personal habits such as cigarette smoking and alcohol consumption

- The presence of certain medical conditions or past medical treatments, including chemotherapy, radiation treatment, or some immune-system suppressing drugs.

- Exposure to cancer-causing agents in the environment (for example, sunlight, radon gas, air pollution, and infectious agents)

An estimated 48,000 cancers are diagnosed yearly in the US that come from occupational causes; this represents approximately 4-10% of total cancer in the United States. It is estimated that 19% of cancers globally are attributed to environmental exposures (including work-related exposures).

HIV-SGD is more prevalent in HIV positive children than HIV positive adults, at about 19% and 1% respectively. Unlike other oral manifestations of HIV/AIDS such as Kaposi sarcoma, oral hairy leukoplakia and oral candidiasis, which decreased following the introduction of highly active antiretroviral therapy (HAART), HIV-SGD has increased.

Human immunodeficiency virus salivary gland disease (abbreviated to HIV-SGD, and also termed HIV-associated salivary gland disease), is swelling of the salivary glands and/or xerostomia in individuals infected with human immunodeficiency virus.

PRP is very rare and similar to SSPE but without intracellular inclusion bodies.

Only 20 patients have been identified since first recognized in 1974.

Although no specific treatment exists, the disease can be managed with anticonvulsants, physiotherapy, etc.

Familial and genetic factors are identified in 5-15% of childhood cancer cases. In <5-10% of cases, there are known environmental exposures and exogenous factors, such as prenatal exposure to tobacco, X-rays, or certain medications. For the remaining 75-90% of cases, however, the individual causes remain unknown. In most cases, as in carcinogenesis in general, the cancers are assumed to involve multiple risk factors and variables.

Aspects that make the risk factors of childhood cancer different from those seen in adult cancers include:

- Different, and sometimes unique, exposures to environmental hazards. Children must often rely on adults to protect them from toxic environmental agents.

- Immature physiological systems to clear or metabolize environmental substances

- The growth and development of children in phases known as "developmental windows" result in certain "critical windows of vulnerability".

Also, a longer life expectancy in children avails for a longer time to manifest cancer processes with long latency periods, increasing the risk of developing some cancer types later in life.

There are preventable causes of childhood malignancy, such as delivery overuse and misuse of ionizing radiation through computed tomography scans when the test is not indicated or when adult protocols are used.

Monocytosis often occurs during chronic inflammation. Diseases that produce such a chronic inflammatory state:

- Infections: tuberculosis, brucellosis, listeriosis, subacute bacterial endocarditis, syphilis, and other viral infections and many protozoal and rickettsial infections (e.g. kala azar, malaria, Rocky Mountain spotted fever).

- Blood and immune causes: chronic neutropenia and myeloproliferative disorders.

- Autoimmune diseases and vasculitis: systemic lupus erythematosus, rheumatoid arthritis and inflammatory bowel disease.

- Malignancies: Hodgkin's disease and certain leukaemias, such as chronic myelomonocytic leukaemia (CMML) and monocytic leukemia.

- Recovery phase of neutropenia or an acute infection.

- Obesity (cf. Nagareddy et al. (2014), Cell Metabolism, Vol. 19, pp 821-835)

- Miscellaneous causes: sarcoidosis and lipid storage disease.

Internationally, the greatest variation in childhood cancer incidence occurs when comparing high-income countries to low-income ones. This may result from differences in being able to diagnose cancer, differences in risk among different ethnic or racial population subgroups, as well as differences in risk factors. An example of differing risk factors is in cases of pediatric Burkitt lymphoma, a form of non-Hodgkin lymphoma that sickens 6 to 7 children out of every 100,000 annually in parts of sub-Saharan Africa, where it is associated with a history of infection by both Epstein-Barr virus and malaria. In industrialized countries, Burkitt lymphoma is not associated with these infectious conditions.

By definition, primary immune deficiencies are due to genetic causes. They may result from a single genetic defect, but most are multifactorial. They may be caused by recessive or dominant inheritance. Some are latent, and require a certain environmental trigger to become manifest, like the presence in the environment of a reactive allergen. Other problems become apparent due to aging of bodily and cellular maintenance processes.

A survey of 10,000 American households revealed that the prevalence of diagnosed primary immunodeficiency approaches 1 in 1200. This figure does not take into account people with mild immune system defects who have not received a formal diagnosis.

Milder forms of primary immunodeficiency, such as selective immunoglobulin A deficiency, are fairly common, with random groups of people (such as otherwise healthy blood donors) having a rate of 1:600. Other disorders are distinctly more uncommon, with incidences between 1:100,000 and 1:2,000,000 being reported.

Revesz syndrome has so far been observed only in children. There is not much information about the disease because of its low frequency in general population and under reporting of cases.

Those affected were born prematurely, and suffered from feeding difficulties and developmental delays. They presented with progressive kidney disease and primary pulmonary hypertension, and ultimately died.

States that agree to the Convention align their domestic rules with the World Anti-Doping Code, which is promulgated by the World Anti-Doping Agency. This includes facilitating doping controls and supporting national testing programmes; encouraging the establishment of "best practice" in the labelling, marketing, and distribution of products that might contain prohibited substances; withholding financial support from those who engage in or support doping; taking measures against manufacturing and trafficking; encouraging the establishment of codes of conduct for professions relating to sport and anti-doping; and funding education and research on drugs in sport.

The International Convention against Doping in Sport is a multilateral UNESCO treaty by which states agree to adopt national measures to prevent and eliminate drug doping in sport.