I actually know a lot about lyme disease from my educational background. In short, the bacteria that causes lyme disease grow in the gut of animals. A tick has four primary life stages (egg, larvae, nymph, adult), each stage is generally marked by them feeding. So the cycle starts with a small rodent such as squirrels and mice and the ticks feed on them at this stage as nymph an picks up the bacteria at this point, it's a nymph and s small as a d on a dime.
Then they feed a second time on a larger mammal (deer/us) which could spread the disease. They are so small that this stage is tricky to see them on you. Then try grow to adults and have their last\third feeding as adults on large animals again and die.
Since the bacteria that causes lyme disease is in their gut, it's crucial to remove the tick from you with tweezers or similar tool, pulling there head an not pinching there body which is what runs the risk of them puking that shit out into you. I cringe when I watch viral "informational" videos on Facebook of people showing "tricks" to remove them using chemicals and other things. Now a days ticks carry so much more nasty disease causing bacteria that borrelia burgdorferi is sadly not the only one of concern anymore.