6.3 Asymptomatic carriers
A related problem for disease eradication is seen in some human infectious diseases, where a carrier state develops. One example is typhoid, where 2–5 per cent of individuals become chronically infected, but without any symptoms of the disease. They excrete the typhoid bacteria continuously into their faeces and act as a persistent reservoir of infection. There have been several notable instances of individuals whose work involved food preparation, who have unwittingly passed the disease to dozens of others via the faecal-oral route.
Another example is the hepatitis B virus (HBV), the major cause of progressive liver disease and hepatic cancers worldwide, which can exist in an asymptomatic carrier state. About two billion people worldwide are infected with HBV, of whom 350 million are carriers.Very rarely, health workers in the UK are discovered to be carriers of HBV when they accidentally infect patients. (Medical students in the UK and people who come into contact with human tissue are usually vaccinated against HBV.) The majority of cases of HBV infection in Western countries are now either sexually transmitted or related to intravenous drug-use; the virus can also be transmitted vertically and post-natal transmission from mother to baby is the principal route in South-East Asia.