Seafilis

There are some pretty dangerous waters out there and if a sailor is not careful enough and gets lured into the depths of debauchery by mermaids and sirens residing there, he shall be met with a foul fate indeed. Seafilis is a disease caused by a parasite that is sexually transmitted and very difficult to cure or get rid off for good. What starts as a night full of fun, can turn into lifetime you share with a being that can influence your mind, decisions and mutate your human core into something strange and otherwordly - a creature neither fit to walk the land or swim the seas permanently, but yearning for both.
Filis is a small little creature resembling a silver coloured tadpole, but in its first stage it is completely invisible to the naked eye (and most people who encounter them in the wild are indeed quite naked). There is no way of avoiding infection besides not boinking below the waves. Abstinence is the only prevention. And if you know one thing about humans, banning horizontal tango is almost as futile as banning alcohol. Which is why this disease spreads extremely quickly, especially in ports and among frequent sea farers. Everyone who has a taste for maritime adventure is at risk for having their mind kidnapped by this parasite for a joyride.
There are several methods of cures, with various degrees of success. Seafilis is very hard to catch early on, especially since the tadpoles can remain dormant for months, gathering energy, slowly feeding off of their hosts. Most people seek help when their sickness has advanced beyond common methods of relief, since there is a lot of stigma and shame associated with it, and some people are simply too naive or ignorant to recognise the warning signs earlier. When seafilis starts to impact your daily life and your body starts to change, you are already loosing the fight with the parasite over your own being and thus you are living on borrowed time. Rich people can usually borrow more than poor.