• Many individuals living in flavivirus endemic areas develop subclinical infection in childhood and gain a degree of protective immunity. Visitors to these areas who first come into contact with arboviruses as adults often die from flavivirus infection since they lack immunity.
  • Some arboviruses, such as dengue, can produce a wide range of symptoms in infected individuals. Depending on the immune response and host defenses, dengue infection may produce inapparent disease, an undifferentiated respiratory illness, dengue fever or dengue hemorrhagic fever/dengue shock syndrome.
  • Infection with a particular flavivirus, dengue, is associated with a particular phenomenon known as "immune enhancement". In the case of dengue, maternally or naturally acquired antibodies against one serotype do not prevent against infection with another serotype. In fact previous exposure to one strain may exacerbate disease caused by exposure to a second strain. The four distinct dengue serotypes are cross-reacting, but not cross-neutralizing. Upon reinfection, the body responds to the first strain with which it was infected and mounts a strong immune response. Uptake of the virus by macrophage cells increases and severe disease (dengue hemorrhagic fever or dengue shock sydrome) may result. This phenomenom is specific to dengue and is known as "immune enhancement".
  • Hepatitis C induces persistent infection and high antibody titers in the chronically infected. Immune responses are unable to clear HCV from the body since most of the virus is bound in virus-antibody complexes. Thus the viral genome remains for years in the liver cells producing a chronic carrier state. But HCV does not overpower the immune system in all cases since the case fatality rate from fulminant hepatitis is less than 1%. The exact nature of the antibody response is unknown because extensive studies have not yet been conducted on the immune response to HCV.