Skip main navigation

Causes of Food Poisoning

What causes food poisoning? We will look at the various causes of food poisoning.
Norovirus that causes 'stomach flu'
© International Culinary Studio

There are many ways that a person can acquire food poisoning.

Non-bacterial causes of food poisoning

Non-bacterial food poisoning is commonly referred to as accidental food poisoning.

The most common non-bacterial risk factors are:

  • Poisonous plants (i.e., mushrooms)
  • Metals
  • Chemicals
  • Equipment
  • Tampering

Bacterial causes of food poisoning

Microorganisms that cause food poisoning are called pathogens or pathogenic bacteria. Food items that have been contaminated by pathogens usually do not show any change in appearance, taste or odour.

A pathogen is a bacterium, virus, or other microorganism that can cause disease. Read more about Pathogens here.

A microbe is a microscopic organism, which may exist in its single-celled form or a colony of cells. Read more about microbes here.

There are other types of bacteria, called food spoilage bacteria. They cause food’s appearance, smell, or taste to change, making it inedible. They can be but are not necessarily poisonous.

Food spoilage bacteria causes food to decay; all foods will eventually decompose. You will see if this if you leave food in a refridgerator for a long time. Visible signs of food spoilage include softening, hardening, leaking, shrivelling, discolouring, bubbling or mouldiness of food. Food can smell mouldy, sulphuric, or putrid, and spoiled canned foods will show signs of leakage and cans may start to bulge.

Note: Not all bacteria are bad. There are “Good” bacteria called probiotics, which exist inside the human colon and assist with digestion, synthesise vitamins, help the immune system and promote good health. Other useful bacteria are used to manufacture cheese, yoghurt and antibiotics.

Probiotics in yoghurt

How do bacteria reproduce

Bacteria reproduce by dividing into two identical cells, which then divide again and again.

The following video shows how bacteria reproduce.

This is an additional video, hosted on YouTube.

Conditions for bacterial growth

Generally, bacteria will multiply every 20 minutes if the environmental conditions are suitable, that is if they have available nutrients, water, and the correct temperature.


Bacteria prefer to multiple in high-protein foods such as meat, poultry, milk products, fish, and eggs.


Bacteria need water to multiply – bacteria present in dry foods remain dormant, multiplying only once water is added.


The optimal temperature for bacteria to multiply is 37℃ / 98.6 ͦF, therefore food items should never be kept at this temperature. Bacteria stops multiplying if temperatures reach 63℃/ 145,4 ͦF and most bacteria are killed at 100℃/212 ͦF. Refrigeration and freezing extend their shelf life as bacterial growth is controlled.

Food Safe Temperatures

Food items should be stored at the following temperatures:

  • Hot cooked foods between 63℃/ 145 ͦF and 65℃/149 ͦF and above
  • Sauces for hot service must be kept above 65°C/149 ͦF
  • Chicken, pork, and minced products must reach above 75°C core temperature.
  • Reheated foods must reach above 75°C/167 ͦF, a temperature of 83°C/182 ͦF is preferred.
  • Ice cream at minus 18℃/64 ͦF
  • Frozen foods at minus 18℃/64 ͦF or below.
  • Chilled products (excluding dairy) at 7℃/44 ͦF
  • Dairy products at 5℃/41 ͦF

Food poisoning bacteria grow well between 5℃ / 41 ͦF and 63℃ / 145 ͦF this is called the danger zone!

For practical reasons, all frozen food items in catering businesses are stored at minus 18℃ / 64 ͦF and chilled items are stored below at 5℃ / 41 ͦF.

Download your own Temperature Conversion Chart

Categories of Food Poisoning:

There are four categories of food poisoning:

1. Bacterial food poisoning

Bacteria enter food items by cross-contamination with other food items. Bacterial contamination commonly occurs with meat products and cheese.

Types of bacteria include:


These are bacteria that need oxygen to multiply. Most bacteria fall into this category.


These are bacteria that do not need oxygen to multiply. Specifically, obligate anaerobes will not multiply in the presence of oxygen and facultative anaerobes can multiply with or without oxygen. For example in the middle of a batch of stew.


Multiplying bacteria are easy to kill. However, some bacteria when exposed to harsh conditions form spores. Spores are round capsules with a hard, outer coat and are resistant to very high temperatures. Spores are very difficult to kill and can remain dormant for a long period of time.

Food items that contain spores must be boiled at least three hours, at very high temperatures.

This is an additional video, hosted on YouTube.

Pathogenic bacteria and toxins

These are bacteria give off waste products that are toxic and poisonous to humans.

2. Viral food poisoning

Viruses enter food items from an infected person that has not washed their hands after using the toilet, smoking, blowing their nose, coughing etc and then handle or prepare food items. A commonly transferred virus is Hepatitis A.

There is currently no evidence that people can catch COVID-19 from food. The virus that causes COVID-19 can be killed at temperatures similar to that of other known viruses and bacteria found in food. Foods such as meat, poultry and eggs should always be thoroughly cooked to at least 70°C / 160°F. Before cooking, raw animal products should be handled with care to avoid cross-contamination with cooked foods. The World Health Organisation WHO 5-Keys to Safer Food provides further information. Coronavirus disease (COVID-19): Food safety and nutrition

3. Chemical food poisoning

Poisonous chemicals enter food items when contaminated by cleaning materials, insecticides, and pesticides.

4. Vegetable food poisoning

Vegetable poisoning occurs by eating naturally poisonous plants such as raw red kidney beans, rhubarb leaves and toadstools.


Make a list of some of the more common food items that you buy from food service establishments, next to these make a list the safe temperatures that they should be kept. For example burger patty 65℃/149 ͦF, this helps to remember the storage temperatures of various items.

© International Culinary Studio
This article is from the free online

Introduction to HACCP for Food Safety

Created by
FutureLearn - Learning For Life

Reach your personal and professional goals

Unlock access to hundreds of expert online courses and degrees from top universities and educators to gain accredited qualifications and professional CV-building certificates.

Join over 18 million learners to launch, switch or build upon your career, all at your own pace, across a wide range of topic areas.

Start Learning now