Why do many diseases such as measles occur only once in a person yet others such as colds occur more than once?

This happens because of our immune system. Our immune system keep record of every attacking microbe. It contains two type of while blood cells firstly is T cell that respond quickly to the attacking microbe. While secondly B cells that recognize those specific cells and fights them off. In addition to that B cells clones itself as memory cells for that disease and will remains in your body for years