Two years ago I went camping on Memorial day and was bit by a tick that was infected with rocky mountain spotted fever bacteria. About a week later I suddenly started getting VERY sick and had an extremely itchy red rash break out all over my body in the space of a couple of hours. I had a bite on my forearm that didn't swell but got deeper and deeper purple and the ring of color grew to about the size of a quarter and itched like crazy. I thought maybe it was a spider bite and I was having a severe histamine reaction. The ER doc said, that doesn't look like a spider bite at all, but have you been bitten by any ticks recently?
They gave me the specific antibiotic to knock out the bacteria, but it can be a deadly illness. Even with the antibiotics, the rash and incredible itch lasted two more weeks. I couldn't even sleep with a blanket on, felt like it was made of sandpaper.
My point here is use Off, use the hell out of it, and reapply. Some things ticks have can very literally kill you.