Dormancy is a characteristic of certain viruses and bacteria where the organism stops growing and multiplying and, for bacteria, enters into a lower metabolic state. This allows the organism to survive in environments which might otherwise be fatal to it.
For example, a virus can survive outside the body of a host in a physically stable state, allowing it to infect another organism who comes into contact with it during the dormancy period. This is a major way in which diseases such as measles are transmitted. In addition, some viruses, like HIV, may temporarily cease multiplying in the body and hide within a body's cells, allowing them to survive both pharmaceuticals and the immune system.
Similarly, in adverse conditions, a bacteria's cell walls and even its cytoplasm may transform into a glass like state which can survive outside a host. In addition, some bacteria, like tuberculosis, can go into periods where it stops multiplying, allowing it to hide in parts of the body that are inaccessible to antibiotics.