Some diseases are caused by infection. Infection is when another organism gets into a person's body and causes harm. There are many kinds of infectious agents: viruses, bacteria, parasites, and fungus. These four kinds of infectious agents cause most infectious disease. There are other kinds though. For example mad cow disease (Bovine spongiform encephalopathy) is caused by prions. Prions are a form of virus that are basically abnormal clumps of proteins. We are not even sure that prions are alive. But they act just like other infectious diseases.