Ehrlichiosis is a nationally notifiable disease in the United States. There have been cases reported in every month of the year, but most cases are reported during April–September. These months are also the peak months for tick activity in the United States.

From 2008-2012, the average yearly incidence of ehrlichiosis was 3.2 cases per million persons. This is more than twice the estimated incidence for the years 2000-2007. The incidence rate increases with age, with the ages of 60–69 years being the highest age-specific years. Children of less than 10 years and adults aged 70 years and older, have the highest case-fatality rates. There is a documented higher risk of death among persons who are immunosuppressed.

Doxycycline and minocycline are the medications of choice. For people allergic to antibiotics of the tetracycline class, rifampin is an alternative. Early clinical experience suggested that chloramphenicol may also be effective, however, in vitro susceptibility testing revealed resistance.

The risk factors associated with BPF are not well known. However, it has been suggested that children under 5 years of age are more susceptible to BPF since they lack serum bactericidal activity against the infection. Older children and adults have much higher titers of bactericidal antibodies, which serve as a protective measure. Also children residing in warmer geographic areas have been associated with a higher risk of BPF infection.

Observations during recent epidemics have suggested chikungunya may cause long-term symptoms following acute infection. This condition has been termed chronic chikungunya virus-induced arthralgia. Long-term symptoms are not an entirely new observation; long-term arthritis was observed following an outbreak in 1979. Common predictors of prolonged symptoms are advanced age and prior rheumatological disease.

During the La Reunion outbreak in 2006, more than 50% of subjects over the age of 45 reported long-term musculoskeletal pain with up to 60% of people reporting prolonged painful joints three years following initial infection. A study of imported cases in France reported that 59% of people still suffered from arthralgia two years after acute infection. Following a local epidemic of chikungunya in Italy, 66% of people reported muscle pains, joint pains, or asthenia at one year after acute infection.

Currently, the cause of these chronic symptoms is not fully known. Markers of autoimmune or rheumatoid disease have not been found in people reporting chronic symptoms. However, some evidence from humans and animal models suggests chikungunya may be able to establish chronic infections within the host. Viral antigen was detected in a muscle biopsy of a person suffering a recurrent episode of disease three months after initial onset. Additionally, viral antigen and viral RNA were found in macrophages in the synovial joint of a person experiencing a relapse of musculoskeletal disease 18 months after initial infection. Several animal models have also suggested chikungunya virus may establish persistent infections. In a mouse model, viral RNA was detected specifically in joint-associated tissue for at least 16 weeks after inoculation, and was associated with chronic synovitis. Similarly, another study reported detection of a viral reporter gene in joint tissue of mice for weeks after inoculation. In a nonhuman primate model, chikungunya virus was found to persist in the spleen for at least six weeks.

The mortality rate of chikungunya is slightly less than 1 in 1000. Those over the age of 65, neonates, and those with underlying chronic medical problems are most likely to have severe complications. Neonates are vulnerable as it is possible to vertically transmit chikungunya from mother to infant during delivery, which results in high rates of morbidity, as infants lack fully developed immune systems. The likelihood of prolonged symptoms or chronic joint pain is increased with increased age and prior rheumatological disease.

The eye gnat ("Liohippelates") was thought to be the cause of the conjunctivitis epidemic which occurred in Mato Grosso do Sul in 1991. These gnats were extracted from the conjunctival secretions of the children who were infected with conjunctivitis. 19 of those children developed BPF following the conjunctivitis. Other modes of transmission include contact with the conjunctival discharges of infected people, ophthalmic instruments which have not been properly sterilized, sharing eye makeup applicators or multiple-dose eye medications.

Nephropathia epidemica ("NE" or epidemic nephropathy) is a type of viral haemorrhagic fever with renal syndrome (HFRS) caused by the Puumala virus. The incubation period is three weeks. Nephropathia epidemica has a sudden onset with fever, abdominal pain, headache, back pain and gastrointestinal symptoms. More severe symptoms include internal hemorrhaging. Although fatal in a small percentage of cases, nephropathia epidemica is generally milder than the HFRS that is caused by hantaviruses in other parts of the world. The bank vole is the reservoir for the virus, which humans contract through inhalation of aerosolised vole droppings.

The majority of infected individuals are asymptomatic or develop only mild symptoms, and the disease is not known to spread from human to human.

This infection is known as "sorkfeber" in Swedish and "myyräkuume" in Finnish (vole fever). In Norway, it is called "musepest" (mouse plague).

Outbreaks of zoonoses have been traced to human interaction with and exposure to animals at fairs, petting zoos, and other settings. In 2005, the Centers for Disease Control and Prevention (CDC) issued an updated list of recommendations for preventing zoonosis transmission in public settings. The recommendations, developed in conjunction with the National Association of State Public Health Veterinarians, include educational responsibilities of venue operators, limiting public and animal contact, and animal care and management.

Contact with farm animals can lead to disease in farmers or others that come into contact with infected animals. Glanders primarily affects those who work closely with horses and donkeys. Close contact with cattle can lead to cutaneous anthrax infection, whereas inhalation anthrax infection is more common for workers in slaughterhouses, tanneries and wool mills. Close contact with sheep who have recently given birth can lead to clamydiosis, or enzootic abortion, in pregnant women, as well as an increased risk of Q fever, toxoplasmosis, and listeriosis in pregnant or the otherwise immunocompromised. Echinococcosis is caused by a tapeworm which can be spread from infected sheep by food or water contaminated with feces or wool. Bird flu is common in chickens. While rare in humans, the main public health worry is that a strain of bird flu will recombine with a human flu virus and cause a pandemic like the 1918 Spanish flu. In 2017, free range chickens in the UK were temporarily ordered to remain inside due to the threat of bird flu. Cattle are an important reservoir of cryptosporidiosis and mainly affects the immunocompromised.

Many viral infections of the central nervous system occur in seasonal peaks or as epidemics, whereas others, such as herpes simplex encephalitis, are sporadic. In endemic areas it is mostly a disease of children, but as the disease spreads to new regions, or nonimmune travelers visit endemic regions, nonimmune adults are also affected.

Paravaccinia virus originates from livestock infected with bovine papular stomatitis. When a human makes physical contact with the livestock's muzzle, udders, or an infected area, the area of contact will become infected. Livestock may not show symptoms of bovine papular stomatitis and still be infected and contagious. Paravaccinia can enter the body though all pathways including: skin contact by mechanical means, through the respiratory tract, or orally. Oral or respiratory contraction may be more likely to cause systemic symptoms such as lesions across the whole body

A person who has not previously been infected with paravaccinia virus should avoid contact with infected livestock to prevent contraction of disease. There is no commercially available vaccination for cattle or humans against paravaccinia. However, following infection, immunization has been noted in humans, making re-infection difficult. Unlike other pox viruses, there is no record of contracting paravaccinia virus from another human. Further, cattle only show a short immunization after initial infection, providing opportunity to continue to infect more livestock and new human hosts.

Some disease-carrying arthropods use cats as a vector, or carrier. Fleas and ticks can carry pathogenic organisms that infect a person with Lyme disease, tick borne encephalitis, and Rocky mountain spotted fever

Allergenic extracts, hormones and vaccines can also cause serum sickness.

Paravaccinia virus is a viral infection of the Parapoxvirus genus of viruses. Human can contract the virus from contact with livestock infected with Bovine papular stomatitis and is common with ranchers, milkers, and veterinarians. Infection will present with fever, fatigue, and lesion on the skin.

Some of the drugs associated with serum sickness are:

- allopurinol

- barbiturates

- captopril

- cephalosporins

- griseofulvin

- penicillins

- phenytoin

- procainamide

- quinidine

- streptokinase

- sulfonamides

- rituximab

- ibuprofen

- infliximab

Meningitis is a very common in children. Newborns can develop herpes virus infections through contact with infected secretions in the birth canal. Other viral infections are acquired by breathing air contaminated with virus-containing droplets exhaled by an infected person. Arbovirus infections are acquired from bites by infected insects (called epidemic encephalitis). Viral central nervous system infections in newborns and infants usually begin with fever. The inability of infants to communicate directly makes it difficult to understand their symptoms. Newborns may have no other symptoms and may initially not otherwise appear ill. Infants older than a month or so typically become irritable and fussy and refuse to eat. Vomiting is common. Sometimes the soft spot on top of a newborn's head (fontanelle) bulges, indicating an increase in pressure on the brain. Because irritation of the meninges is worsened by movement, an infant with meningitis may cry more, rather than calm down, when picked up and rocked. Some infants develop a strange, high-pitched cry. Infants with encephalitis often have seizures or other abnormal movements. Infants with severe encephalitis may become lethargic and comatose and then die. To make the diagnosis of meningitis or the diagnosis of encephalitis, doctors do a spinal tap (lumbar puncture) to obtain cerebrospinal fluid (CSF) for laboratory analysis in children.

Feline zoonosis are the viral, bacterial, fungal, protozoan, nematode and arthropod infections that can be transmitted to humans from the domesticated cat, "Felis catus". Some of these are diseases are reemerging and newly emerging infections or infestations caused by zoonotic pathogens transmitted by cats. In some instances, the cat can display symptoms of infection (these may differ from the symptoms in humans) and sometimes the cat remains asymptomatic. There can be serious illnesses and clinical manifestations in people who become infected. This is dependent on the immune status and age of the person. Those who live in close association with cats are more prone to these infections. But those that do not keep cats as pets are also able to acquire these infections because of the transmission can be from cat feces and the parasites that leave their bodies.

People can acquire cat-associated infections through bites, scratches or other direct contact of the skin or mucous membranes with the cat. This includes 'kissing' or letting the animal lick the mouth or nose. Mucous membranes are easily infected when the pathogen is in the mouth of the cat. Pathogens can also infect people when there is contact with animal saliva, urine and other body fluids or secretions, When fecal material is unintentionally ingested, infection can occur. Feline zooinosis can be acquired by a person by inhalation of aerosols or droplets coughed up by the cat.

In the United States, forty percent of homes have at least one cat. Some contagious infections such as campylobacteriosis and salmonellosis cause visible symptoms of the disease in cats. Other infections, such as cat scratch disease and toxoplasmosis, have no visible symptoms and are carried by apparently healthy cats.

Although epidemiologic characteristics of the adenoviruses vary by type, all are transmitted by direct contact, fecal-oral transmission, and occasionally waterborne transmission. Some types are capable of establishing persistent asymptomatic infections in tonsils, adenoids, and intestines of infected hosts, and shedding can occur for months or years. Some adenoviruses (e.g., serotypes 1, 2, 5, and 6) have been shown to be endemic in parts of the world where they have been studied, and infection is usually acquired during childhood. Other types cause sporadic infection and occasional outbreaks; for example, epidemic keratoconjunctivitis is associated with adenovirus serotypes 8, 19, and 37. Epidemics of febrile disease with conjunctivitis are associated with waterborne transmission of some adenovirus types, often centering on inadequately chlorinated swimming pools and small lakes. ARD is most often associated with adenovirus types 4 and 7 in the United States. Enteric adenoviruses 40 and 41 cause gastroenteritis, usually in children. For some adenovirus serotypes, the clinical spectrum of disease associated with infection varies depending on the site of infection; for example, infection with adenovirus 7 acquired by inhalation is associated with severe lower respiratory tract disease, whereas oral transmission of the virus typically causes no or mild disease. Outbreaks of adenovirus-associated respiratory disease have been more common in the late winter, spring, and early summer; however, adenovirus infections can occur throughout the year.

"Ad14 (for adenovirus serotype 14), has caused at least 140 illnesses in New York, Oregon, Texas and Washington, according to a report from the Centers for Disease Control and Prevention. The illness made headlines in Texas in September 2007, when a so-called "boot camp flu" sickened hundreds at Lackland Air Force Base in San Antonio. A 19-year-old trainee died."

Several adenoviruses, including Ad5, Ad9, Ad31, Ad36, Ad37, and SMAM1, have at least some evidence of causation of obesity in animals, adipogenesis in cells, and/or association with human obesity. To date, the most thorough investigations have been conducted for adenovirus serotype 36 (Adv36).

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.

Host tropism is the infection specificity of certain pathogens to particular hosts and host tissues. This type of tropism explains why most pathogens are only capable of infecting a limited range of host organisms.

Researchers can classify pathogenic organisms by the range of species and cell types that they exhibit host tropism for. For instance, pathogens that are able to infect a wide range of hosts and tissues are said to be amphotropic. Ecotropic pathogens, on the other hand, are only capable of infecting a narrow range of hosts and host tissue. Knowledge of a pathogen's host specificity allows professionals in the research and medical industries to model pathogenesis and develop vaccines, medication, and preventative measures to fight against infection. Methods such as cell engineering, direct engineering and assisted evolution of host-adapted pathogens, and genome-wide genetic screens are currently being used by researchers to better understand the host range of a variety of different pathogenic organisms.

Immunodeficiency or immunosuppression can be caused by:

- Malnutrition

- Fatigue

- Recurrent infections

- Immunosuppressing agents for organ transplant recipients

- Advanced HIV infection

- Chemotherapy for cancer

- Genetic predisposition

- Skin damage

- Antibiotic treatment leading to disruption of the physiological microbiome, thus allowing some microorganisms to outcompete others and become pathogenic (e.g. disruption of intestinal flora may lead to "Clostridium difficile" infection

- Medical procedures

- Pregnancy

- Ageing

- Leukopenia (i.e. neutropenia and lymphocytopenia)

The lack of or the disruption of normal vaginal flora allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.

Based on the low variability exhibited among known SARS-CoV-2 genomic sequences, the strain is thought to have been detected by health authorities within weeks of its emergence among the human population in late 2019. The earliest case of infection currently known is thought to have been found on 17 November 2019. The virus subsequently spread to all provinces of China and to more than 150 other countries in Asia, Europe, North America, South America, Africa, and Oceania. Human-to-human transmission of the virus has been confirmed in all of these regions. On 30 January 2020, SARS-CoV-2 was designated a Public Health Emergency of International Concern by the WHO, and on 11 March 2020 the WHO declared it a pandemic.

The basic reproduction number (R0) of the virus has been estimated to be between 1.4 and 3.9. This means that each infection from the virus is expected to result in 1.4 to 3.9 new infections when no members of the community are immune and no preventive measures are taken. The reproduction number may be higher in densely populated conditions such as those found on cruise ships. Many forms of preventive efforts may be employed in specific circumstances in order to reduce the propagation of the virus.

There have been about 82,000 confirmed cases of infection in mainland China. While the proportion of infections that result in confirmed cases or progress to diagnosable disease remains unclear, one mathematical model estimated that on 25 January 2020 75,815 people were infected in Wuhan alone, at a time when the number of confirmed cases worldwide was only 2,015. Before 24 February 2020, over 95% of all deaths from COVID-19 worldwide had occurred in Hubei province, where Wuhan is located. As of 17 April 2020, the percentage had decreased to 2.1%.

As of 17 April 2020, there have been 2,234,109 total confirmed cases of SARS-CoV-2 infection in the ongoing pandemic. The total number of deaths attributed to the virus is 153,379. Many recoveries from confirmed infections go unreported, but at least 567,695 people have recovered from confirmed infections.

The common cold is the most common human disease and affects people all over the globe. Adults typically have two to five infections annually, and children may have six to ten colds a year (and up to twelve colds a year for school children). Rates of symptomatic infections increase in the elderly due to declining immunity.

Native Americans and Inuit are more likely to be infected with colds and develop complications such as otitis media than Caucasians. This may be explained by issues such as poverty and overcrowding rather than by ethnicity.

The common cold is generally mild and self-limiting with most symptoms generally improving in a week. Half of cases go away in 10 days and 90% in 15 days. Severe complications, if they occur, are usually in the very old, the very young, or those who are immunosuppressed. Secondary bacterial infections may occur resulting in sinusitis, pharyngitis, or an ear infection. It is estimated that sinusitis occurs in 8% and ear infection in 30% of cases.

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).