But it wasn't until 1796 that a country doctor from England named Edward Jenner formally vaccinated a child against the disease. It, however, would be nearly two centuries later before smallpox was eradicated worldwide (the last known case occurred in Somalia in 1977). Its banishment (except for samples held in Russian and American laboratories) has been heralded as one of the most significant medical achievements in history. Today, safe, hair-thin needles deliver nearly painless injections, while some vaccines can be given orally with drops or nasally through a spray. Invaders such as bacteria, viruses and other pathogens display "signs" on their surface called antigens that signal immune system cells to action. A more targeted approach to threats comes from your adaptive immune system, which responds to specific antigens.