Infectious diseases are caused by a variety of organisms including bacteria, viruses, parasites and fungi. Some of these may enter the body and cause what is known as a systemic infection, others may cause more superficial infections. In most cases our immune system will become activated and attempt to rid the body of these foreign organisms. This reaction takes time. As a result the infecting organism can severely damage the body and may even cause death before it can successfully be destroyed by the immune response.