Five hepatitis viruses have been identified, A, B, C, D, E. The parenterally transmitted hepatitis viruses, hepatitis B and hepatitis C, are of most concern to health care workers as they are highly infectious in blood.
Hepatitis B has a DNA genome. The virus replicates in the liver and is shed in large amounts into the blood where its presence is prolonged. The virus lodges in the hepatocytes and can cause two forms of chronic infection. One is chronic persistent infection, which doesn’t cause liver damage but can result in hepatocellular carcinoma and the other is chronic active hepatitis which causes destruction of the liver tissue which leads to cirrhosis or liver failure.
Hepatitis C has a single-strand RNA (ssRNA) genome. It causes a milder form of acute hepatitis than hepatitis B but chances of developing chronic hepatitis are very much higher. This can lead to chronic liver failure or hepatocellular carcinoma.
Four human retroviruses have been identified. All infect the CD4 bearing cells. These viruses were only discovered in the 1980’s when it became possible to culture T-cells in vitro. HIV 1 is responsible for the AIDS pandemic and HIV 2, which is of lower virulence is largely confined to West Africa.
The pathogenesis of AIDS starts with the infection of the CD4 cells, dendritic cells, and macrophages. About 2 to 4 weeks later a flu-like illness occurs. This is followed by a specific immune response and seroconversion takes place. An asymptomatic phase follows, which can last from 2 to 10 years. During this phase most of the virus is cleared from the peripheral blood but viral replication continues in lymphoid tissue. This leads to a gradual loss of CD4 cells and the destruction of the microenvironment of lymphoid tissue. The infected individual starts to suffer from prodromal disorders. Finally a state of gross immunodefiency is reached and the individual has full-blown AIDS.
In the prodromal and final stages of the disease, individuals are prone to a host of opportunistic infections. These can include protzoal infections (pneumocystis carinii pneumonia, enteritis, encephalitis), fungal infections (candida, cryptococcus neoformans), viral infections (herpes simplex, varicella zoster, cytomegalo virus, leukoencephaly), and bacterial infections (tuberculosis, atypical mycobacterium avium intracellulare, salmonella streptococcus) (1).