What causes hair fungus?
Your question is not clear. If you want to know about the causes of fungal disease in the scalp such as ringworm (a common scalp fungus in children), it is most often spread from animals. The animals themselves are immune to the disease, but act as carriers. It can spread from person to person as well, so it travels through families and is highly contagious. People with compromised immune systems are susceptible to these infections as well. For reason that are not clear, the elderly and children are frequently impacted more than adults and this suggests some ‘weakness’ in the immune system (children may have not built an immunity to the fungus).
If you want to know about other fungus, please be more specific.