Ticks are parasites that latch on to many mammals to feed blood off of them. They appeared in Old school and The Deep End.
History[]
Old School[]
After dinner when they got back to their cabin, the chaperone of the cabin, Robert Jefferson told all the kids to check each other for ticks. While they were checking for them, Mr. Jefferson said that each person was responsible for his bunkmate and Greg was the bunk bunkmate of Mr. Jefferson, it meant he also had to check Mr. Jefferson for ticks. But Greg just pretended to check for ticks because he mentions that Mr. Jefferson has a lot of hair and that a colony of ticks might be living in there.
Also, when the kids were eating, Jordan found a tick on the back of Julian's neck and everybody freaked out. Mr. Jefferson said all to calm down and he took Julian to the nurse.
The Deep End[]
Greg asked Rodrick to check him for ticks when they got back to the camp from a 'hike'. Rodrick, wanting to prank Greg, showed him a picture of a large tick online and told him that this tick was on his back. He then said that the tick was probably on his back for a long time and was about to burst, before showing the picture. He later told Greg that it was a joke, but Greg still thought that there was something on his back for the rest of the day.