Infections - bacterial and viral Many human illnesses are caused by infection with either bacteria or viruses. Bacteria are tiny single-celled organisms. They are among the most successful life forms on the planet, and range in habitat from ice slopes to deserts. but some are responsible for a range of illnesses. These disease-causing varieties are called pathogenic. Many bacterial infections can be treated successfully with appropriate antibiotics. A virus is an even tinier micro-organism that can only reproduce inside a host's living cell. It is very difficult to kill a virus. That's why some of the most serious communicable diseases known to medical science are viral in origin.