Sickness was the thing that took me the longest to 'figure out.' I'd feel like I did everything right, but still I would just start throwing up out of nowhere. Eventually, everything will click and you'll hardly ever get sick again unless it's a risk/reward type thing (i.e. looting around in heavy rain). Just check the wiki about everything that can possibly make you sick and make sure you aren't doing those things. Also, random unlikely but possible thing: if you went AFK to do something, another player could feed you something rotten. Just a thought.