Leprosy, also known as Hansen's disease, used to mean a one-way ticket to banishment. But once medicine trained its sights on wiping out what might be the most ancient disease to afflict humans, it has become treatable and even accepted.
Viruses are big jerks that invade regular cells and hold them hostage, making you sick while they're doing it. Learn everything you ever needed to know about viruses, including how the common cold works.
Our current understanding of allergies are that they are defensive measures launched unnecessarily against benign intruders like pollen, which is mistaken by the immune system as a threatening foreign invader. The concept of immunotherapy is based on this logic: By exposing the immune system to small consistent doses of a benign substance like pollen the immune system won't launch a full-on attack.