It can actually get sick! There's no real known factor into what causes it, it could hang around with contagious people all the time and be fine. But if it ever does get 'sick', it gets sorta sluggish and sleepy... sometimes it 'sneezes' and sprays black goop from that filter. It's just like ichor not tar in this instance, but thankfully doesn't carry any germs or anything! Ironically this is one of the best times to be around it because it can't be bothered to rip your arm off. Just don't try to pet it or anything unless it knows you it can still DO that but needs provoking.