Food can become contaminated with harmful substances for various reasons, leading to food poisoning. Some common causes include:

1. Bacteria: Such as Salmonella, E. coli, and Listeria monocytogenes.
2. Viruses: Like norovirus and hepatitis A.
3. Parasites: For example, Toxoplasma gondii.
4. Chemical toxins: From pesticides, heavy metals (like lead or mercury), or naturally occurring toxins in certain plants and fish.
5. Allergens: Proteins that can trigger allergic reactions in sensitive individuals.

To prevent food poisoning, it's important to practice good hygiene, proper food handling, cooking to safe temperatures, and storing food correctly. If you suspect food poisoning, seek medical attention if symptoms are severe or persist.