Prophylaxis is the central idea in preventative medicine. People usually think medical treatment helps sick people to get healthy. Prophylactic treatment is helpful in a different way. Primary prophylaxis tries to stop healthy people from getting sick. Secondary prophylaxis tries to stop people who are sick from getting worse.
Prophylaxis may also be used as a synonym for birth control. Condoms are prophylactic, because when used properly they can prevent pregnancy or sexually transmitted diseases. In the strict meaning and medical meaning of the word "profylaxe", it only means prevention of DISEASE, not of birth. For example, oral contraceptive is not a profylaxe, because it does not prevent disease.