Drawn

Drawn has been a cataclysmic sickness, almost a curse, inflicted on civilization by some long dead Sea Monster.

Symptoms

The first thing to be wary of is dreams of the sea: nightmares of something far far away, in the darkest depths, noticing you. You can keep quiet about this, though others may question the bags under your eyes and your aversion to rain.
About two weeks to a month later, as the dreams increase in frequency and intensity, you develop a fever. This is more difficult to hide. At this point, people will become suspicious.
Sometime within the next month, you will begin to vomit up sea water. You will have no control over this, no warning, and no good way of hiding it. Once you vomit up seawater, it is too late. Drawn is spreading.
Something is coming. You don't know what, you don't know when. At this point, others will know. Your family may try to protect you, rationalizing that you as a society must stick together, that you must not turn on each other and become monsters. But altruism is fickle. Everyone is scared for themselves and those they love, so if history is any reputable reference, your neighbours, and even the law, will likely demand you to be quarantined: dropped off on some remote island or set a sail far away with anyone else who is sick. You are doomed, but they are scared.
They are scared because of the final symptom. When the nightmares become reality and whatever Sea Monster has been searching for you comes, and destroys everything that is near.

Treatment

Powerful priests have been known to be able to cure someone of Drawn, but it is risky. If the process goes wrong, they instead could attract the scent of the monster. Many times, attempts to rid someone of the disease has led to its quickened spread.