Food may become contaminated with microbes at many points during its production, processing, transportation, storage, distribution, and preparation for consumption. The degree of hazard and points of maximum danger vary, depending on the types of contamination, and on the food and its method of production, including handling and processing procedures. Food contamination is often caused by faulty preparation. WHO data indicates that only a few factors are responsible for a large proportion of foodborne disease episodes. Common errors include: (a) preparation of food long before consumption; (b) prepared food left too long at a temperature permitting bacterial proliferation; (c) inadequate heating; (d) cross-contamination between cooked and raw food; (e) infected or "colonized" person handling the food. Also some raw foods are frequently contaminated and consequently increase the risk of foodborne disease.
Microbiologically-contaminated food is responsible for a high proportion of diarrhoeal and other infectious diseases, particularly in the developing world. Foodborne diarrhoea is a major contributor to malnutrition. As much as 70% of diarrhoeal diseases in the developing countries are now believed to be of foodborne origin.
In many countries, poultry meat, and in some cases eggs, has been identified as important sources of foodborne agents. Even present day technology cannot guarantee the production of pathogen-free poultry. This often results in an important contamination of the poultry meat with foodborne pathogens such as Salmonella, Campylobacter and Listeria. In some studies, up to 100% of the tested samples have been found to be contaminated.
Unlike meat and dairy products, vegetables and fruit do not promote the growth of disease-causing bacteria. However, their surfaces may have adhered farm manure that is contaminated with pathogens. They may also have come in contact with E. coli and Salmonella through faeces of wild animals and handling by people with dirty hands. Listeria monocytogenes, another pathogen, is naturally present in some soil, and it is not uncommon to find it on fresh produce.
Microbiologically-contaminated foods are particularly dangerous during pregnancy. For example, toxoplasmosis and listeriosis can be fatal for the foetus or cause severe malformations. Some foodborne disease may also leave chronic effects such as congenital malformations, heart and vascular disease, as well as disorder of the renal and immune systems.