Most types of spores that land on the skin or are breathed into the lungs do not cause illness. A few varieties only infect persons who have one of the following: Immune system weakness (for example, from cancer treatment or AIDS) Lung problems (for example, asthma or cystic fibrosis) Heart problems (for example, heart transplant patients) Neurological problems (for example, people with Alzheimer's disease or Parkinson's disease) However, if the immune system is working properly, any part of the body can be infected. The most common sites of infection are the lungs, skin, kidneys, liver, brain and heart. Infections can be caused by fungi that people breathe in through their airways (nasal cavity and throat), eat with their food (mouth and esophagus), touch with their hands (skin), drink through their stomachs (gastrointestinal tract), or inject into their bodies with needles (intravenous drug users). Fungi are microscopic plants that grow along with other organisms in soil and decompose organic material. They may also live alone or in groups inside animals or humans.