According to WebMD, the organism that spreads oral thrush can be spread by kissing, but this is not a major way oral thrush is spread. This is because the organism that causes oral thrush, the candida fungus, is already present in most people's mouths and in other parts of their bodies. The organism only causes disease when the immune system is weakened or the body's normal microbial balance is disrupted.
WebMD states that oral thrush is caused by the same organism that causes vaginal yeast infections. In the case of oral thrush, however, infections are most common in babies and small children. Adults are also vulnerable if their immune systems are depressed because of a disease like HIV or through the use of certain medications, such as immunosuppressants. Other medications that create a risk of oral thrush are birth control pills and antibiotics. Physical conditions such as smoking, wearing dentures or dry mouth also increase risk.
According to WebMD, oral thrush usually begins suddenly. It is easily detected by the presence of raised white lesions, most commonly on the tongue and inner cheeks. These are often accompanied by pain or difficulty swallowing. In the case of people with depressed immune systems, the infection can spread beyond the mouth to the skin, lungs and elsewhere on the body.