Just about every animal has its intimate companions, its parasites. Linguists have suggested that one of the very first words in the human vocabulary was a term for flea or tick.
Ticks are small external parasites found on birds, reptiles, and mammals.
They are not insects but close relatives of mites and spiders. At each stage they need a blood meal before going on to the next stage. With the blood they pick up any microbes that might be on circulating in their temporary host and carry them to their next meal, thus spreading disease. This, of course, is not desirable since whatever tick climbs aboard you most probably got its last meal from something not human. The biggest problem ticks have is finding an appropriate host for their next meal.