Some bacteria, viruses and fungi can cause illnesses and diseases. Diseases that can spread easily from one person to another are called contagious diseases. Examples of contagious diseases are measles, mumps, conjunctivitis and smallpox. It can spread through air, water, direct contact and through vectors such as mosquitoes, flies and rats. Non-contagious diseases cannot be spread from one person to another. Examples of non-contagious diseases are tooth decay, stomach upset, etc.