However, there are groups which are very difficult to reach for example prostitutes, ethnic minorities in the mountain villages and uneducated men working a long way from home. The usual explanation is that the difference is the result of sexual behaviour. It was also believed that HIV was easier to transmit if one already carried other sexually transmissible diseases. But these theories only explain part of the great differences in the distribution of the infection. Another explanation may be that various subsidiary groups of HIV virus exist.
Some research suggest that the various subsidiary groups vary as regards their infectiousness.
In Western Europe and the USA, the persons primarily affected by HIV initially were homosexuals, people receiving blood transfusions and blood products and also drug abusers using hypodermic syringes. The number of heterosexual persons has been rather low but is now, comparatively speaking, increasing compared with the other groups. In Africa and Thailand it is primarily heterosexuals who are infected. The first HIV patient in Thailand was infected with HIV-1 B, that is the type most common in Europe. The patient was most probably infected by a tourist.