Viral hepatitis Hepatitis A Hepatitis A is found throughout the world and manifests as large outbreaks and epidemics associated with fecal contamination of water and food sources. Hepatitis A viral infection is predominant in children ages 5–14 with rare infection of infants. Infected children have little to no apparent clinical illness, in contrast to adults in whom greater than 80% are symptomatic if infected. Infection rates are highest in low resource countries with inadequate public sanitation and large concentrated populations. In such regions, as much as 90% of children younger than 10 years old have been infected and are immune, corresponding both to lower rates of clinically symptomatic<unk>  and outbreaks. The availability of a childhood vaccine has significantly reduced infections in the United States, with incidence declining by more than 95% as of 2013. Paradoxically, the highest rates of new infection now occur in young adults and adults who present with worse clinical illness. Specific populations at greatest risk include: travelers to endemic regions, men who have sex with men, those with occupational exposure to non-human primates, individuals with clotting disorders who have received clotting factors, individuals with a history of chronic liver<unk>  where co-infection with hepatitis A can lead to fulminant hepatitis, and intravenous drug users (rare). Hepatitis B HBV replication Hepatitis B is the most common cause of viral hepatitis in the world with more than 240 million chronic carriers of the virus, 1 million of whom are in the United States. In approximately two-thirds of patients who develop acute hepatitis B infection, no identifiable exposure is evident. Of those acutely infected, 25% become lifetime carriers of the virus. Risk of infection is highest among intravenous drug users, individuals with high-risk sexual behaviors, healthcare workers, individuals with a history of multiple transfusions, organ transplant patients, dialysis patients and newborns infected during the birthing process. Close to 780,000 deaths in the world are attributed to hepatitis B. The most endemic regions are in sub-Saharan Africa and East Asia, where as many as 10% of adults are chronic carriers. Carrier rates in developed nations are significantly lower, encompassing less than 1% of the population. In endemic regions, transmission is thought to be associated with exposure during birth and close contact between young infants. Hepatitis C HepC replication Chronic hepatitis C is a major cause of liver cirrhosis and hepatocellular carcinoma. It is a common medical reason for liver transplantation due to its severe complications. It is estimated that 130–180 million people in the world are affected by this<unk>  representing a little more than 3% of the world population. In the developing regions of Africa, Asia and South America, prevalence can be as high as 10% of the population. In Egypt, rates of hepatitis C infection as high as 20% have been documented and are associated with iatrogenic contamination related to schistosomiasis treatment in the 1950s–1980s. Currently in the United States, approximately 3.5 million adults are estimated to be infected. Hepatitis C is particularly prevalent among people born between 1945 and 1965, a group of about 800,000 people, with prevalence as high as 3.2% versus 1.6% in the general U.S. population. Most chronic carriers of hepatitis C are unaware of their infection status. The most common mode of transmission of hepatitis C virus is exposure to blood products via blood transfusions (prior to 1992) and intravenous drug injection. A history of intravenous drug injection is the most important risk factor for chronic hepatitis C. Other susceptible populations include individuals with high-risk sexual behavior, infants of infected mothers, and healthcare workers. Hepatitis D The hepatitis D virus causes chronic and fulminant hepatitis in the context of co-infection with the hepatitis B virus. It is primarily transmitted via non-sexual contact and via needles. Susceptibility to hepatitis D differs by geographic region. In the United States and Northern Europe, populations at risk are intravenous drug users and individuals who receive multiple transfusions. In the Mediterranean, hepatitis D is predominant among hepatitis B virus co-infected individuals. Hepatitis E Similar to Hepatitis A, hepatitis E manifests as large outbreaks and epidemics associated with fecal contamination of water sources. It accounts for more than 55,000 deaths annually with approximately 20 million people worldwide thought to be infected with the virus. It affects predominantly young adults, causing acute hepatitis. In infected pregnant women, Hepatitis E infection can lead to fulminant hepatitis with third trimester mortality rates as high as 30%. Individuals with weakened immune systems, such as organ transplant recipients, are also susceptible. Infection is rare in the United States but rates are high in the developing world (Africa, Asia, Central America, Middle East). Many genotypes exist and are differentially distributed around the world. There is some evidence of hepatitis E infection of animals, serving as a reservoir for human infection. Alcoholic hepatitis 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<unk>  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. Non-alcoholic steatohepatitis Non-alcoholic steatohepatitis (NASH) is projected to become the top reason for liver transplantation in the United States by 2020, supplanting chronic liver<unk>  due to hepatitis C. About 20–45% of the U.S. population have NAFLD and 6% have NASH. The estimated prevalence of NASH in the world is 3–5%. Of NASH patients who develop cirrhosis, about 2% per year will likely progress to hepatocellular carcinoma. Worldwide, the estimated prevalence of hepatocellular carcinoma related to NAFLD is 15–30%. NASH is thought to be the primary cause of cirrhosis in approximately 25% of patients in the United States, representing 1–2% of the general population.
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