When people learn that they have genital herpes, the first reaction is often a feeling of isolation - of being alone with the problem. But if you have genital herpes, you are far from alone. Infection with HSV-2 (the virus which is the most common cause of genital herpes) is a lot more common than you might think. One in five people in the USA - 20 per cent of the population - are infected with HSV-2. In other countries, information on the number of people who are infected with HSV-2 has often been collected in certain groups of individuals. For example, a number of studies have been carried out in pregnant women to determine how many have the HSV-2 virus.   Infection with HSV-2 is more common in Africa, where between 27 and 71% of adults are infected. HSV-2 infection is most common in people aged 20 to 30 years. The number of people infected with HSV-2 is increasing rapidly. In the USA, it rose by 30 per cent between 1980 and 1994. There is a similar trend in other parts of the world. Yet four in five people who are infected with HSV-2 do not even know it - either because they and their doctors do not recognise the symptoms, or because they are infected with the virus but never suffer from any symptoms.