Necessary condition

An event A is said to be a necessary condition for event B whenever B cannot occur without the occurrence of A.

A necessary condition is a condition that must be true in order for a result to be true. For example:
 * "Living is a necessary condition for breathing, because you must be alive to breathe, if you are breathing then you are definitely alive, and if you are not living, you are not breathing."
 * "Viral infection is a necessary condition for polio, because you must be infected with poliovirus to get polio, if you have polio you have definitely been infected with poliovirus, and if you have not been infected with poliovirus, you do not have polio."

The other type of condition is a sufficient condition.