Food poisoning occurs when a food has been contaminated or defected before ingestion, and, as such, causes illness or severe complications in the body. Food poisoning is caused by an infectious organism such as Bacterias parasites (salmonella or Escherichia coli), and viruses (norovirus). Some foods are easily vulnerable to infections if not properly cooked, stored and handled, this food include unpasteurized milk, raw fish meat, and poultry, ready to eat foods (soft cheese, packed sandwich, hot dogs, salads). 
Generally, foods get contaminated if not cooked thoroughly, especially livestock. Also, exposure to moisture, leaving the food in a poorly hygienic environment, poor storage of refrigerated foods and infected food handlers. Basically, foods are naturally contaminated; it depends on the level of exposure to what bacteria, Precautionary hygienic methods or the efficiency of the immune system before symptoms can occur. Food poisoning can be resolved on its own, but some people still need to see a doctor. 
The symptoms are not cryptic to discern; they appear within one or days and sometimes a few hours to a week of contamination. Here are the symptoms to determine if you are suffering from food poisoning.