A surge in “chemsex” parties, where some people spend days getting high on drugs and having sex with scores of partners, is refuelling epidemics of HIV among gay men in European towns and cities, doctors say. Despite much higher risks of contracting the virus that causes AIDS, as well as other sexually transmitted infections (STIs), users search online “hook-up” apps like Grindr for tags such as “high and horny” or “party and play” to find others wanting drug-heightened and often anonymous and unprotected sex. The result, AIDS experts say, is that in cities across Europe, HIV is spreading rapidly among men who have sex with men – leading to concentrated epidemics in hard-to-reach groups.