Your lungs will slowly drown you from the inside.
Siren's kiss is a quite poetical name for a serious illness. It is a sexually transmitted disease of unknown origin, which symptoms show themselves after it's already too late to do something about them. So, while it is quite rare, it's always deadly. Apart from that, there are three main reasons why is it so feared: 1) No one really knows where it comes from or what causes it apart from the fact that one must be sexually active to develop the condition. Anyone can be the carrier and the moment someone has it, it usually spreads like wildfire. Majority of healers believe it comes from contaminated water, but the popular opinion is that it comes from having sex with the "fishfolk" which makes them and anyone associating with them a frequent target of ostracism and violence. 2) The very early symptoms are the same as for the common cold, which is probably the most frequent illness in Saphiros - runny nose, sneezing, sore throat, headaches and fatigue. That makes it practically impossible to detect it while it is still treatable. The moment your lips turn blue (hence the name for the illness), your chest gets heavy and your lungs start to burn from the bottom up, it is too late for anything, and chances of full recovery are one in a thousand. 3) The way you die is painful and ugly. Your lungs are slowly soaked in liquid, causing such pain it's like you are burning from the inside, which only gets more painful with the intensifying cough. The breathing gets more and more difficult each day and it can take months before your lungs collapse completely and the illness finally takes you. It is genuinely believed that normal drowning is actually a nicer way to go and many infected commit suicide the moment they are sure they have it (especially poor people out of shame or for not being able to afford a good healer).