Yellow fever is common in tropical and subtropical areas of South America and Africa. Worldwide, about 600 million people live in endemic areas. The WHO estimates 200,000 cases of disease and 30,000 deaths a year occur; the number of officially reported cases is far lower.

Vaccination is recommended for those traveling to affected areas, because non-native people tend to develop more severe illness when infected. Protection begins by the 10th day after vaccine administration in 95% of people, and had been reported to last for at least 10 years. WHO now states that a single dose of vaccination is sufficient to confer lifelong immunity against yellow fever disease." The attenuated live vaccine stem 17D was developed in 1937 by Max Theiler. The World Health Organization (WHO) recommends routine vaccinations for people living in affected areas between the 9th and 12th month after birth.

Up to one in four people experience fever, aches, and local soreness and redness at the site of injection. In rare cases (less than one in 200,000 to 300,000), the vaccination can cause yellow fever vaccine–associated viscerotropic disease, which is fatal in 60% of cases. It is probably due to the genetic morphology of the immune system. Another possible side effect is an infection of the nervous system, which occurs in one in 200,000 to 300,000 cases, causing yellow fever vaccine-associated neurotropic disease, which can lead to meningoencephalitis and is fatal in less than 5% of cases.

The Yellow Fever Initiative, launched by WHO in 2006, vaccinated more than 105 million people in 14 countries in West Africa. No outbreaks were reported during 2015. The campaign was supported by the GAVI Alliance, and governmental organizations in Europe and Africa. According to the WHO, mass vaccination cannot eliminate yellow fever because of the vast number of infected mosquitoes in urban areas of the target countries, but it will significantly reduce the number of people infected.

In March 2017, WHO launched a vaccination campaign in Brazil with 3.5 million doses from an emergency stockpile. In March 2017 the WHO recommended vaccination for travellers to certain parts of Brazil.

About 15–20% of hospitalized Lassa fever patients will die from the illness. The overall mortality rate is estimated to be 1%, but during epidemics, mortality can climb as high as 50%. The mortality rate is greater than 80% when it occurs in pregnant women during their third trimester; fetal death also occurs in nearly all those cases. Abortion decreases the risk of death to the mother. Some survivors experience lasting effects of the disease, and can include partial or complete deafness.

Because of treatment with ribavirin, fatality rates are continuing to decline.

Globally, symptomatic HAV infections are believed to occur in around 1.4 million people a year. About 114 million infections (asymptomatic and symptomatic) occurred all together in 2015. Acute hepatitis A resulted in 11,200 deaths in 2015. Developed countries have low circulating levels of hepatovirus A, while developing countries have higher levels of circulation. Most adolescents and adults in developing countries have already had the disease, thus are immune. Adults in midlevel countries may be at risk of disease with the potential of being exposed.

Treatment is similar to hepatitis B, but due to its high lethality, more aggressive therapeutic approaches are recommended in the acute phase. In absence of a specific vaccine against delta virus, the vaccine against HBV must be given soon after birth in risk groups.

Pregnant women who contract HEV are at significant risk of developing fulminant hepatitis with maternal mortality rates as high as 20–30%, most commonly in the third trimester . A 2016 systematic review and meta-analysis of 47 studies that included 3968 people found maternal case-fatality rates (CFR) of 20.8% and fetal CFR of 34.2%; among women who developed fulminant hepatic failure, CFR was 61.2%.

Alcoholic hepatitis (AH) in its severe form has a one-month mortality as high as 50%. Most people who develop AH are men but women are at higher risk of developing AH and its complications likely secondary to high body fat and differences in alcohol metabolism. Other contributing factors include younger age <60, binge pattern drinking, poor nutritional status, obesity and hepatitis C co-infection. It is estimated that as much as 20% of people with AH are also infected with hepatitis C. In this population, the presence of hepatitis C virus leads to more severe disease with faster progression to cirrhosis, hepatocellular carcinoma and increased mortality. Obesity increases the likelihood of progression to cirrhosis in individuals with alcoholic hepatitis. It is estimated that a high proportion of individuals (70%) who have AH will progress to cirrhosis.

The hepatitis E virus causes around 20 million infections a year. These result in around three million acute illnesses and as of 2010, 57,000 deaths annually. It is particularly dangerous for pregnant women, who can develop an acute form of the disease that is lethal in 30% of cases or more. HEV is a major cause of illness and of death in the developing world and disproportionate cause of deaths among pregnant women. Hepatitis E is endemic in Central Asia, while Central America and the Middle East have reported outbreaks.

Lábrea fever is a coinfection or superinfection of hepatitis D or delta virus and hepatitis B (HBV). The infection by delta virus may occur in a patient who already has the HBV, or both viruses may infect at the same time a previously uninfected patient. Delta virus can only multiply in the presence of HBV, therefore vaccination against HBV prevents infection. Thus, American and Brazilian scientists have determined that the delta virusa, virus, which is a small circular RNA virus, is normally unable to cause illness by itself, due to a defect. When it is combined with HBV, Lábrea hepatitis may ensue. The main discovery of delta virus and HBV association was done by Dr. Gilberta Bensabath, a leading tropical virologist of the Instituto Evandro Chagas, of Belém, state of Pará, and her collaborators.

Infected patients show extensive destruction of liver tissue, with steatosis of a particular type (microsteatosis, characterized by small fat droplets inside the cells), and infiltration of large numbers of inflammatory cells called "morula cells", comprised mainly by macrophages containing delta virus antigens.

In the 1987 Boca do Acre study, scientists did an epidemiological survey and reported delta virus infection in 24% of asymptomatic HBV carriers, 29% of acute nonfulminant hepatitis B cases, 74% of fulminant hepatitis B cases, and 100% of chronic hepatitis B cases. The delta virus seems to be endemic in the Amazon region.

The bacterium that causes typhoid fever may be spread through poor hygiene habits and public sanitation conditions, and sometimes also by flying insects feeding on feces. Public education campaigns encouraging people to wash their hands after defecating and before handling food are an important component in controlling spread of the disease. According to statistics from the United States Centers for Disease Control and Prevention (CDC), the chlorination of drinking water has led to dramatic decreases in the transmission of typhoid fever in the United States.

The number of people infected by Lassa range from 100,000 to three million a year, with up to 5,000 deaths per year in West Africa alone. In certain areas such as Sierra Leone and Liberia, 10-16% of people admitted to hospital have the virus. The case fatality rate for those who are hospitalized for the disease is about 15-20%. Research in Guinea showed a twofold increase risk of infection for those living in close proximity to someone with infection symptoms within the last year.

Lassa has been linked to high risk areas near the western and eastern extremes of West Africa. These areas cannot be well defined by any known biogeographical or environmental breaks. However, it is relatively common in parts of West Africa where the multimammate rat is common, particularly Guinea (Kindia, Faranah and Nzerekore regions), Liberia (mostly in Lofa, Bong, and Nimba counties), Nigeria (everywhere) and Sierra Leone (typically from Kenema and Kailahun districts). It is present but less common in the Central African Republic, Mali, Senegal and other nearby countries, and less common yet in Ghana and the Democratic Republic of the Congo. Benin had its first confirmed cases in 2014, and Togo had its first confirmed cases in 2016.

The spread of Lassa outside of West Africa has been very limited. Twenty to thirty cases have been described in Europe, cited as being caused by importation through infected individuals. These causes found outside of West Africa were found to have a high fatality risk because of the delay of diagnosis and treatment due to being unaware of the risk associated with the symptoms. These imported cases have not manifested in larger epidemics outside of Africa due to a lack of human to human transmission in hospital settings. The exception of this happened in 2003 when a healthcare worker became infected before the patient showed clear symptoms.

The study of the epidemiology of Lassa fever is complicated by a lengthy incubation period, which may be up to three weeks. Incubation periods as long as Lassa fever may affect spatial clustering of the disease by limiting the understanding of the incidence and distribution of the disease. The spatial clustering for this disease is still in development as a lack of easy-available diagnosis, limited public health surveillance infrastructure, and high clustering of incidence near high intensity sampling make for an incomplete look at the impact of Lassa in this region.

In 2004, an estimated 350 million individuals were infected worldwide. National and regional prevalences range from over 10% in Asia to under 0.5% in the United States and Northern Europe.

Routes of infection include vertical transmission (such as through childbirth), early life horizontal transmission (bites, lesions, and sanitary habits), and adult horizontal transmission (sexual contact, intravenous drug use).

The primary method of transmission reflects the prevalence of chronic HBV infection in a given area. In low prevalence areas such as the continental United States and Western Europe, injection drug abuse and unprotected sex are the primary methods, although other factors may also be important. In moderate prevalence areas, which include Eastern Europe, Russia, and Japan, where 2–7% of the population is chronically infected, the disease is predominantly spread among children. In high-prevalence areas such as China and South East Asia, transmission during childbirth is most common, although in other areas of high endemicity such as Africa, transmission during childhood is a significant factor. The prevalence of chronic HBV infection in areas of high endemicity is at least 8% with 10–15% prevalence in Africa/Far East. As of 2010, China has 120 million infected people, followed by India and Indonesia with 40 million and 12 million, respectively. According to World Health Organization (WHO), an estimated 600,000 people die every year related to the infection.

In the United States about 19,000 new cases occurred in 2011 down nearly 90% from 1990.

In the United States in 1991, the mortality rate for hepatitis A was estimated to be 0.015% for the general population, but ranged up to 1.8 -2.1 % for those aged 50 and over which were hospitalized with icteric hepatitis. The risk of death from acute liver failure following HAV infection increases with age and when the person has underlying chronic liver disease.

Young children who are infected with hepatitis A typically have a milder form of the disease, usually lasting 1–3 weeks, whereas adults tend to experience a much more severe form of the disease.

The rate of hepatitis C in immunosuppressed people is higher than the normal population. This is particularly true in those with human immunodeficiency virus infection, recipients of organ transplants and those with hypogammaglobulinemia. Infection in these people is associated with an unusually rapid progression to cirrhosis.

The WHO lists 25 diseases for which vaccines are available:

1. Measles

2. Rubella

3. Cholera

4. Meningococcal disease

5. Influenza

6. Diphtheria

7. Mumps

8. Tetanus

9. Hepatitis A

10. Pertussis

11. Tuberculosis

12. Hepatitis B

13. Pneumoccocal disease

14. Typhoid fever

15. Hepatitis E

16. Poliomyelitis

17. Tick-borne encephalitis

18. Haemophilus influenzae type b

19. Rabies

20. Varicella and herpes zoster (shingles)

21. Human papilloma-virus

22. Rotavirus gastroenteritis

23. Yellow fever

24. Japanese encephalitis

25. Malaria

26. Dengue fever

virus DNA persists in the body after infection, and in some people the disease recurs. Although rare, reactivation is seen most often following alcohol or drug use, or in people with impaired immunity. HBV goes through cycles of replication and non-replication. Approximately 50% of overt carriers experience acute reactivation. Males with baseline ALT of 200 UL/L are three times more likely to develop a reactivation than people with lower levels. Although reactivation can occur spontaneously, people who undergo chemotherapy have a higher risk. Immunosuppressive drugs favor increased HBV replication while inhibiting cytotoxic T cell function in the liver. The risk of reactivation varies depending on the serological profile; those with detectable HBsAg in their blood are at the greatest risk, but those with only antibodies to the core antigen are also at risk. The presence of antibodies to the surface antigen, which are considered to be a marker of immunity, does not preclude reactivation. Treatment with prophylactic antiviral drugs can prevent the serious morbidity associated with HBV disease reactivation.

The most common cause of hepatitis is viral. Although they are classified under the disease hepatitis, these viruses are not all related.

Compared with adults, infection in children is much less well understood. Worldwide the prevalence of hepatitis C virus infection in pregnant women and children has been estimated to 1–8% and 0.05–5% respectively. The vertical transmission rate has been estimated to be 3–5% and there is a high rate of spontaneous clearance (25–50%) in the children. Higher rates have been reported for both vertical transmission (18%, 6–36% and 41%). and prevalence in children (15%).

In developed countries transmission around the time of birth is now the leading cause of HCV infection. In the absence of virus in the mother's blood transmission seems to be rare. Factors associated with an increased rate of infection include membrane rupture of longer than 6 hours before delivery and procedures exposing the infant to maternal blood. Cesarean sections are not recommended. Breastfeeding is considered safe if the nipples are not damaged. Infection around the time of birth in one child does not increase the risk in a subsequent pregnancy. All genotypes appear to have the same risk of transmission.

HCV infection is frequently found in children who have previously been presumed to have non-A, non-B hepatitis and cryptogenic liver disease. The presentation in childhood may be asymptomatic or with elevated liver function tests. While infection is commonly asymptomatic both cirrhosis with liver failure and hepatocellular carcinoma may occur in childhood.

This condition most commonly occurs after the administration of a horse origin biological agent such as equine-derived antiserum, and usually occurs 4–10 weeks after the event. Diseases that have been vaccinated against using equine-origin antiserum, resulting in subsequent Theiler's disease, include: African horse sickness, Eastern and Western Equine Encephalitis, "Bacillus anthracis", tetanus antitoxin, "Clostridium perfringens", "Clostridium botulinum", "Streptococcus equi" subspecies "equi", Equine influenza, Equine herpesvirus type 1, pregnant mare's serum, and plasma. Although it occurs sporadically, It appears to be spreadable within a premises, and there have been outbreaks occurring on farms involving multiple horses over several months. In the Northern hemisphere it is most common between August to November. It is seen almost exclusively in adult horses, and lactating broodmares given tetanus antitoxin post foaling may be more susceptible.

Sanitation and hygiene are important to prevent typhoid. Typhoid does not affect animals other than humans. Typhoid can only spread in environments where human feces are able to come into contact with food or drinking water. Careful food preparation and washing of hands are crucial to prevent typhoid. Industrialization, and in particular, the invention of the automobile, contributed greatly to the elimination of typhoid fever, as it eliminated the public health hazards associated with having horse manure in the public street which led to large number of flies.

"Hepatitis C" (originally "non-A non-B hepatitis") is caused by hepatitis C virus (HCV), an RNA virus that is a member of the Flaviviridae family. HCV can be transmitted through contact with blood (including through sexual contact if the two parties' blood is mixed) and can also cross the placenta. Hepatitis C usually leads to chronic hepatitis, culminating in cirrhosis in some people. It usually remains asymptomatic for decades. Patients with hepatitis C are susceptible to severe hepatitis if they contract either hepatitis A or B, so all persons with hepatitis C should be immunized against hepatitis A and hepatitis B if they are not already immune, and avoid alcohol. HCV viral levels can be reduced to undetectable levels by a combination of interferon and the antiviral drug ribavirin. The genotype of the virus is the primary determinant of the rate of response to this treatment regimen, with genotype 1 being the most resistant.

Hepatitis C is the most common chronic blood-borne infection in the United States.

This depends on the degree of hepatocellular necrosis that has occurred. Decreases in the SDH and prothrombin time along with improvement in appetite are the best positive predictive indicators of recovery. GGT may remain elevated for weeks even if the horse is recovering. Horses that survive for greater than one week and that continue to eat usually recover. Cases with rapid progression of clinical signs, uncontrollable encephalopathy, haemorrhage or haemolysis have a poor prognosis. Horses that display clinical signs have a mortality rate of 50–90%.

Currently, no vaccine against relapsing fever is available, but research continues. Developing a vaccine is very difficult because the spirochetes avoid the immune response of the infected person (or animal) through antigenic variation. Essentially, the pathogen stays one step ahead of antibodies by changing its surface proteins. These surface proteins, lipoproteins called variable major proteins, have only 30–70% of their amino acid sequences in common, which is sufficient to create a new antigenic "identity" for the organism. Antibodies in the blood that are binding to and clearing spirochetes expressing the old proteins do not recognize spirochetes expressing the new ones. Antigenic variation is common among pathogenic organisms. These include the agents of malaria, gonorrhea, and sleeping sickness. Important questions about antigenic variation are also relevant for such research areas as developing a vaccine against HIV and predicting the next influenza pandemic.

Genotypes 1 and 2 are restricted to humans and often associated with large outbreaks and epidemics in developing countries with poor sanitation conditions. Genotypes 3 and 4 infect humans, pigs, and other animal species and have been responsible for sporadic cases of hepatitis E in both developing and industrialized countries.

In the United Kingdom, the Department for Environment, Food and Rural Affairs said that the number of human hepatitis E cases increased by 39% between 2011 and 2012.

The pathogenic agent is found everywhere except New Zealand. The bacterium is extremely sustainable and virulent: a single organism is able to cause an infection. The common source of infection is inhalation of contaminated dust, contact with contaminated milk, meat, or wool, and particularly birthing products. Ticks can transfer the pathogenic agent to other animals. Transfer between humans seems extremely rare and has so far been described in very few cases.

Some studies have shown more men to be affected than women, which may be attributed to different employment rates in typical professions.

“At risk” occupations include:

- Veterinary personnel

- Stockyard workers

- Farmers

- Sheep shearers

- Animal transporters

- Laboratory workers handling potentially infected veterinary samples or visiting abattoirs

- People who cull and process kangaroos

- Hide (tannery) workers