Radiation hormesis is when low doses of ionizing radiation are beneficial, stimulating repair mechanisms that protect against disease.
Simply...Low doses of radiation are slightly damaging your bodies cells and DNA causing it to react by increasing production of stronger healthier cells, also causing the effects of aging to slow down.
The immune system also reacts and increases in strength, this causes your body to fight off illnesses, it is not the radiation that is killing the illness, it is your body. This is the reason Radiation Hormesis is so beneficial, it makes your body do what it is supposed to do.
The radiation helps two fold. Fungus is a simple organism that does not have the means to repair itself quick enough from the low doses of radiation, so if your illness is caused by a fungus, the effect the radiation has will be increased.