Food can become contaminated by: not cooking it thoroughly (particularly meat), not correctly storing food that needs to be chilled at below 5°C, not sufficiently reheating previously cooked food, and eating food that has passed its “use by” date.
Other ways include:
- leaving cooked food for too long at warm temperatures
- someone who is ill or who has dirty hands touching the food
- the spread of bacteria between contaminated foods (cross-contamination)
Source: Food poisoning @ NHS Choices