Fungal infection thrives on dead organic material. Tending to spread rapidly, it especially likes warm, moist areas (and absolutely loves it when you eat sugar).
Parasite infection and fungal infection seem to occur together, so if one is present, be sure to check (or treat) for the other.
When it comes down to it, fungus infections are the result of a depressed or compromised immune system.
The people who are most susceptible to fungal infection are those who take frequent antibiotic treatments, use birth control pills, are obese, or have diseases such as diabetes, Epstein-Barr, or HIV.