Microorganisms such as viruses, bacteria and fungi are everywhere, including in and on your body. More than 200 species of bacteria live in your mouth, in your digestive and respiratory tracts and on your skin. In general, the relationship between you and this array of microbes is mutually beneficial. You provide nutrition, protection and transportation for them, while they stimulate your immune system, synthesize essential vitamins and help protect against harmful viruses and bacteria.

But your relationship to microorganisms in the world at large is more complex. Some microbes are highly beneficial, but others — such as those that cause malaria and meningitis — can be deadly. For that reason, your immune system works to repel harmful invading organisms while maintaining a balance between "good" and "bad" microbes that normally inhabit your body.

But sometimes these protective mechanisms fail. Oral thrush and other Candida infections occur when your immune system is weakened by disease or drugs such as prednisone or other corticosteroids, or when antibiotics disturb the natural balance of microorganisms in your body.

• Meningitis

• Sjogren's syndrome

• Yeast infection

• Xerostomia

These illnesses may make you more susceptible to oral thrush infection:

• Chronic mucocutaneous candidiasis. Usually affecting children younger than age 3, this group of rare disorders is marked by a chronic Candida infection of the mouth and fingernails as well as skin on the scalp, trunk, hands and feet. Scaly, crusted lumps known as granulomas also may develop in the mouth or on the nails and skin. Adults occasionally develop the disorder — usually as a result of a tumor on the thymus gland (thyoma).

• HIV/AIDS. The human immunodeficiency virus (HIV) — the virus that causes AIDS — damages or destroys the cells of your immune system, making you more susceptible to opportunistic infections your body would normally resist. One of these opportunistic infections is oral thrush. Thrush is rare in the early stage of AIDS, usually appearing only when counts of helper T cells — one of the key cells in your immune system — fall below 350. Although oral thrush is the least serious of the fungal infections that can affect people with HIV, it may be an indication that HIV is worsening. Candida esophagitis, which occurs when thrush spreads to the esophagus, generally develops when T cell counts are 200 or less and is considered an AIDS-defining illness — an opportunistic illness that indicates a person with HIV is developing AIDS.

• Cancer. If you're dealing with cancer, your immune system is likely to be weakened both from the disease and from treatments such as chemotherapy and radiation. A weakened immune system increases the chances you may develop Candida infections such as thrush.

• Diabetes mellitus. If you don't know you have diabetes or the disease isn't well controlled, your saliva may contain large amounts of sugar. This can encourage the growth of Candida.

• Vaginal yeast infections. Vaginal yeast infections are caused by Candida albicans — the same fungus that causes oral thrush. Although a yeast infection isn't dangerous, a pregnant woman can pass the fungus to her baby during delivery. As a result, her newborn may develop oral thrush within the first several weeks after birth.

• Dry mouth (xerostomia). This occurs when the salivary glands in your mouth don't produce enough moisture. Dry mouth disrupts the balance of normal microorganisms in your mouth, increasing your risk of oral thrush. Although not itself a disease, dry mouth can be a symptom of certain illnesses, including Sjogren's syndrome — an autoimmune disease that causes an extremely dry mouth and eyes. Bone marrow transplants, stress or anxiety, depression, and certain nutritional deficiencies also can cause a dry mouth. So can chemotherapy, radiation to the head and neck area and hundreds of medications — especially antidepressants, pain and high blood pressure drugs, tranquilizers, diuretics and antihistamines.

• Anxiety

• chemotherapy

• high blood pressure

• Depression

• Meningitis

• Nicotine dependence

• Sjogren's syndrome

• Yeast infection

• Xerostomia

Certain lifestyle factors also can make you more likely to develop oral thrush:

• Smoking. Chronic hyperplastic candidiasis is a type of thrush that occurs in people who smoke. It causes creamy white patches on the insides of the cheeks and tongue, along with persistent nodules or lumps. Red patches also may sometimes appear, which can be a sign that cancerous changes are occurring in the cells.

• Wearing dentures. People who wear dentures — especially if they don't fit properly — sometimes develop a type of thrush marked by redness and swelling.

Anyone can develop oral thrush, but the infection is especially common in infants and toddlers whose immune systems aren't yet fully developed. In addition, babies can pass the infection to their mothers during breast-feeding.

You're also more likely to develop oral thrush if you:

• Are an older adult

• Have a compromised immune system

• Use corticosteroids, antibiotics or birth control pills

• Smoke

• Wear dentures