Food adulteration refers to intentionally reducing the quality of food by adding cheaper or harmful substances. Common adulterants include watering down milk, using artificial colors and preservatives, or substituting ingredients like oleomargarine for butter. To avoid illness from contaminated or adulterated food and water, it is important to select foods carefully, cook foods thoroughly, avoid raw or undercooked meat/fish, and be cautious of street vendors. Boiling, chemical disinfection, or water filters can be used to treat water and make it safe for drinking.