Milk and dairy products and eggs can be contaminated in various ways, in which case they constitute a hazard to public health. Milk contains few bacteria when it leaves the udder of the healthy cow, but is liable to contamination from the exterior of the animal, especially the exterior of the udder and adjacent parts. Micro-organisms of manure, soil and water may enter from this source. Automatic milking apparatus and various milk utensils may add contaminants after the milk leaves the udder. Contamination can also occur from the tanker truck and various utensils and equipment at the market milk plant, cheese factory, condensery or other processing plant. Other possible sources of contamination are the hands of the milker or other dairy workers, and flies which may add spoilage organisms or pathogens.
Although the majority of freshly laid eggs are sterile inside, the shells become contaminated by faecal matter from the hen, by washing water if the eggs are washed, and by handling. Micro-organisms, including pathogenic Salmonella, can penetrate a cracked shell and enter the egg, so causing food poisoning.
An egg-rich diet can cause elevated blood cholesterol and heart disease.