Botulism is caused by Clostridium botulinum bacteria, which can be found in soil, dust and river or sea sediments.

The bacterium itself is not harmful, but it can produce highly poisonous toxins when deprived of oxygen (such as in closed cans or bottles, stagnant soil or mud, or occasionally the human body). These toxins are some of the deadliest substances known to science.

C. botulinum can produce seven different types of toxin, which are classified as types A through to G. Types A, B, E and F are poisonous to humans, with Type F the most toxic and type B the least toxic.

These toxins stop nerve cells releasing a chemical called acetylcholine, which is needed for signals to be sent from your nerves to your muscles. This lack of acetylcholine can cause paralysis that gets gradually worse and, if left untreated, can be fatal.

Botulism is usually divided into three main types, depending on the way the bacteria get into the body. These are described below.

Food-borne botulism

Food-borne botulism can occur when food becomes contaminated with infected soil. If food is not properly canned, preserved or cooked, any C. botulinum bacteria in the food can start producing toxins that are highly poisonous if eaten.

Modern food production techniques use high-temperature processes that ensure all bacteria are killed. Therefore, cases of food botulism are virtually unheard of in the UK.

However, faults during the food processing or storage process can lead to very occasional outbreaks and there is a risk of homemade food becoming contaminated if proper storage and cooking precautions are not taken.

Following an outbreak of food-borne botulism in 1989, where contaminated hazelnut yoghurt affected 26 people and resulted in one death, there have been another nine cases of this type of botulism in England and Wales up to 2013. All nine cases were caused by eating homemade food or food produced in other countries.

Wound botulism

Wound botulism occurs when a wound becomes infected with the C. botulinum bacteria. Most cases are caused by injecting or sniffing drugs contaminated with the bacteria. Once inside your body, the bacteria can produce the poisonous toxins.

From 2000 to 2013 there were 147 cases of wound botulism in people who inject drugs in England and Wales. All of these occurred in people injecting heroin into their muscles.

Most heroin users inject the drug into their veins, but some users prefer to inject into muscles. This may be because they are trying to disguise signs of heroin injection ("track marks") or their veins have been damaged by previous heroin abuse.

Injecting heroin directly into muscle increases the risk of botulism because it provides a better environment for the C. botulinum to produce toxins. Even very small infections can cause severe illness, as the bacteria continuously produce toxins that build up to high levels.

Using clean needles and not sharing needles with others will not prevent wound botulism because it is not the needle that is infected but the heroin itself.

Most heroin in the UK comes from opium poppies grown in Asia or the Middle East, particularly Afghanistan. The opium is cooked and the morphine extracted before being treated with chemicals to produce heroin. The drugs are then 'cut' with bulking materials, such as starch and lactose. The heroin can become contaminated at any point during this process.

Some cases of wound botulism have also been linked to nasal tissue damage caused by snorting cocaine. The damaged tissue can become infected. However, no cases of this kind have ever been recorded in the UK.

Infant botulism

Infant botulism occurs when a baby ingests spores of the Clostridium botulinum bacteria. The spores make their way to the intestine (part of the digestive system) where they begin to produce toxins.

Clostridium botulinum bacteria spores are harmless to older children and adults. After about the age of one year the baby develops defences that counter the effects of the spores.

Honey and corn syrup have been known to cause some cases of infant botulism, although some cases have been linked to ingesting contaminated soil or dust. No definitive cause is found in many cases.

There were only 16 cases of infant botulism in England and Wales between 1975 and 2013, although 10 of these occurred from 2007 onwards. None of the 16 cases were fatal.

Other possible causes

It is possible to be infected with the botulinum toxin in other ways, although these have never happened in the UK. Possibilities include:

• an incorrect injection of botulinum toxin – botulinum toxin injections (such as Botox) can be used to make wrinkles less obvious, however there have been a few cases where a version of the medication that was not intended for human use caused botulism
• breathing in the toxin when it is in the form of a gas – this does not happen naturally and would need to be deliberately prepared, for example by terrorists
• drinking the toxins in water – the normal treatment process for mains water supply would de-activate the toxins, so the water would need to be deliberately contaminated, for example by terrorists 

Botulinum toxins cannot be spread from person to person.