It is both cases:
- Mosquito requires that you have caught it in the Swamp, then there is a 50% chance of having the Mosquito at night, or something else
- Parasite Worm requires that you have at least 1 parasite with you (infected), and then again, chance based. The more parasites, the bigger the chance
- Big Parasite Worm happens after 3 events with normal Parasite Worm, but at the same chance of the normal Parasite Worm