How do you contaminate food?

The most likely way is by not washing your hands thoroughly before handling food. The worst way some people contaminate food is by not washing their hands after using the lavatory, particularly after producing a number two in the lavatory and after wiping their behind with toilet paper and then after leaving lavatory handling the food with their same hands used to wipe their behind. This can make whoever eats that food very ill. So can not washing your hands after urinating.
Food may also get contaminated by vermin, especially in very hot humid weather, if insects like flies land on it, particularly if those same flies have previously landed on human or animal excreta. Other insects like cockroaches can also contaminate food if ever allowed to climb on to it. Another serious way you can contaminate food is if you allow rats to get anywhere near where it is lying or where it is stored.
Some types of meat, particularly chicken, can be contaminated if not cooked properly before eating so causing somebody to get salmonella poisoning. Double freezing is another way of contaminating food. If something is deep frozen in the fridge and taken out to prepare for eating but then after it has thawed, you make the mistake of putting some of it back in the deep-freeze again for future use, the next time you take it out it could be contaminated and whoever eats it could get ill.