The body is coated internally and externally with fungi. It sounds awful but, like the bacteria in the body, they are essential to our well-being. They prevent foreign fungi, which may be very toxic to our system, from gaining a foothold by what is called competitive inhibition. Fungi thrive on sugars and other nutrients and our own fungi are not harmful to us. If we kill them off by the use of antifungal agents or poor diet, unhealthy fungi will find themselves with more food to eat because of the absence of the good guys and will multiply quicker, producing their toxic effects.
Yeasts are fungi. The most infamous is monilia, more commonly known as Candida which produces the symptoms of thrush. Thrush is controlled due to the phenomenon of cross-species competitive inhibition. To put it more simply, antibiotics and unhealthy lifestyle kill off our own fungi and bacteria leaving more food for the tougher varieties. Candida lives in a high percentage of our bodies and will not cause us a problem unless we destroy its competitors. This is why people in immunocompromised states and those who have taken antibiotics can end up with thrush.
As in the case of antibacterial agents, resistant strains occur and we are seeing more virulent, aggressive and faster-replicating yeast and fungi species developing due to the over-prescribing of antifungal and anti-Candida preparations.