C. difficile (C. diff) is an anaerobic bacterium. This means that it does not need oxygen to survive and multiply (reproduce). Therefore it usually survives well in the large intestine, where there is very little oxygen available.

C. diff does not usually affect healthy children and adults, because the healthy 'good' bacteria in the intestine keep it in check. However, some antibiotics can interfere with this healthy balance of bacteria. When this happens, C. diff can begin to multiply and produce toxins (poisons). At this point, a person is said to be 'infected' with C. diff.

Once C. diff bacteria start to produce toxins, the bacteria can spread easily. This is because they can produce spores even when exposed to unfavourable conditions, such as being outside of the body. Spores are what bacteria produce so they can multiply.

C. diff spores leave the body in an infected person's diarrhea. The spores can then contaminate their surroundings, such as toilets, bedclothes, skin and clothing. The spores can also be spread through the air (during bed-making for example). The spores are able to survive for a long time outside of the body unless they are destroyed through very thorough cleaning.

This means anyone who comes into contact with contaminated surfaces can easily spread the infection. The spores can then infect other people by entering the body through the mouth.

People most vulnerable to a C. diff infection are those who:

• have been treated with broad spectrum antibiotics (antibiotics that can treat different types of bacteria),
• have had to stay for a long time in a healthcare setting, such as a hospital,
• are over 65 years old,
• have a serious underlying illness or condition,
• have a weakened immune system, or
• have had numerous enemas or gut surgery.

Most infections occur in places where many people are taking antibiotics, and in close contact with each other, such as hospitals and nursing homes. A number of precautions can be put in place to help reduce the spread of the infection (see the prevention section).

People who have C. diff naturally in their gut cannot spread the bacteria unless the bacteria start producing toxins. This is why many people have the bacteria in their body, but do not have any symptoms.