Oral thrush, candidiasis, is fungal infection, which is common with a compromised immune system, such as with cancer, and a secondary condition due to Mucocitis from the radiation and chemo. As Cheyrl said, it feeds on sugar, and is basically an uncontrolled yeast infection. Avoiding sugar, breads, alcohol, excess fruits, etc. and anything that irritates the mouth such acidic foods, spices, sharp textured foods, hot foods. Thrush can go to the esophagus, stomach, and if untreated, it can elsewhere throughout the body via the blood system, and become invasive or dessiminated candidiasis, a potentially deadly condition. The mouth is also vulnerable to spread of infection due to any ulcers present into the blood. The treatment is with an antifungal, not antibiotic, such as the Nystatin rinse, but Fluconozole (Diflucan), clotromozole are more effective, and both are often used. Impeccable oral care is important by brushing after each meal, before bed with a soft brush rinsing often with baking soda rinse w/salt if it doesn't burn, flossing, and changing your tooth brushes often to avoid reinfection.