Aside from wearing nearly nothing outside in the cold/damp weather (unless you'd rather prefer having nothing on), there's always eating foods that don't agree with you. Preferably if they're rotten foods, that might hasten illness. There's motion sickness and hanging around people in hospitals (or on the streets) who aren't well. You can easily catch a cold from another person.
When it's cold outside then being the warm-blooded creature that you are, you'll need to stay warm to survive. Doing otherwise will turn on your body's defenses and let the cold win. An easy way would be to take a dip in freezing lake water. Have you any ice ponds nearby? Also avoiding medicines, including the annual flu shot, will help to keep your body in disarray.