Viruses are not true living cells and are much smaller than bacteria. Viruses do not multiply in food. They do multiply in cells in various organs or tissues of the body. For example, Hepatitis A virus replicates in cells of the liver. It is quite difficult to isolate viruses on or in food.
Yeasts are much larger than bacteria.
However, Candida albicans , which causes thrush in infants, is the exception. The growth of some types of yeast is used to produce fermented food products. Undesired growth of some yeast in food products causes spoilage. Molds are larger than bacteria. The presence of mold in food can be seen as a cottony, powdery or fuzzy patch on the surface of food that may be gray, white or highly colored.