Seven factors in which harmful substances could contaminate food
http://www.health.gov.au/internet/publications/publishing.nsf/Content/ohp-enhealth-manual-atsi-cnt-l~ohp-enhealth-manual-atsi-cnt-l-ch3~ohp-enhealth-manual-atsi-cnt-l-ch3.8
antibiotics: antibiotics can get into the animals flesh and milk.If people drink the milk or eat the meat,they can absorb the antibiotics.This can lead to antibiotic resistance.This means that when a person is sick and needs antibiotics,they don't work.
artificial hormones :This hormones lead to deformed calves and mastitis. It can make the levels of some chemicals in humans too high, which has been linked to colon and breast cancer.
Animal by_products :these products can make animals I'll such as foot-and-mouth disease and mad cow disease.There is also a risk of salmonella bacteria that can make people I'll.
Pesticides:There are some toxic pesticides that are stored in body fat and can make people I'll.