Border-crossing groups often include sex workers. Some studies suggest that busy land border crossings and international fishing ports have higher rates of HIV and sexually transmitted infections than other locations. Sex workers are particularly vulnerable to HIV and represent the most significant core group for transmission to the rest of the population through their clients. The main factors influencing the spread of HIV include the number of clients a day and the proportion of men who visit sex workers regularly. The absence of condom use increases transmission even more.
Sex work continues to be illegal in many countries so prevention remains difficult.
Men who have sex with men represent one of the highest risk groups because of the transmission probabilities associated with unprotected anal sex. Many of these men may also have sex with women and so help spread HIV to their female partners. The region has been significantly affected by injecting drug users, the first community to be affected in most countries in South and Southeast Asia. Drug users are rapidly switching from non-injecting drugs to injecting drugs, forcing previously undetectable levels of HIV to increase significantly.