5. Lyme Disease
Very tiny ticks infected with very nasty bacteria pass on Lyme disease while sucking up a blood dinner. You may not even notice the tick, but you will notice the resulting infection. The early infection involves headaches, muscle and joint pain, and other flu-like symptoms. As it progresses, headaches increase along with severe joint pain and swelling and shooting pains in hands and feet.
Lyme disease requires special testing to identify. Because it is slowly (but steadily) spreading west and early stages mimic flu, it is frequently misdiagnosed.