Only around 200 types of fungi can cause disease in humans and most of these are opportunistic infections that cause problems only in people who are unhealthy in some other way.

Candidiasis is also known as thrush, or sometimes more broadly called a yeast infection. It is caused by fungi of the Candida species but is not the same as brewer's or baker's yeast. Candida infections affect the mouth and genitals although it can sometimes spread to other body organs. Infections of the oesophagus and lungs are common in patients suffering from the later stages of HIV/AIDS.

The fungus Candida albicans lives for most of the time as self-contained cells which reproduce asexually. In this form they cause little or no harm and can be found in the mouth, large intestine, vagina or skin of 50% of people. If conditions change, the candida are able to grow thread-like hyphae that penetrate the skin and cause an infection that appears as a creamy-white layer. In very rare cases, the Candida can spread into the blood and cause damage to the heart, eyes, kidneys and liver.

Candida albicans can be spread from one person to the next by direct contact or in fluids like saliva. It is so common that many people carry the fungus without any symptoms and it is usually some change in the person's health that triggers the infection to become a problem.

Candida grows where the skin is moist and warm. It is often found in the mouth, vagina, penis and between the buttocks. For most of the time any infection is prevented from taking hold by the skin's tough outer layer, the body's immune system and friendly commensal bacteria. Any reduction in these defences can result in candidiasis.

Patients with a weakened immune system are vulnerable and candidiasis can develop in AIDS sufferers who often get serious infections of the lungs. Patients on cancer therapy or drugs to suppress their immune system following an organ transplant are also susceptible to opportunistic infections like candida.

Antibiotics reduce the numbers of commensal bacteria in the mouth and vagina which allows the fungus to become established. Oral candida is common in new-born babies because they do not have commensal bacteria in their mouths and can be infected during birth. Nappy rash sometimes provides enough damage to the surface of the skin between the buttocks to provide the opportunity for candida to infect.

Although unpleasant, most Candida infections are not life-threatenting and can be treated quite successfully with anti-fungal medicines. These are effective because they specifically target the fungus cells and destroy their cell walls. Creams, powders or suppositories applied directly to the infected area may also include an anti-inflammatory steroid to reduce soreness. Oral treatments may only need one dose to treat the fungus. Infected areas should be kept clean and dry, with baby's nappies changed regularly and anti-fungal powders applied.