Smallpox The smallpox virus causes the once common disease smallpox, now effectively eliminated by vaccination. Unlike bacteria, viruses are entirely parasitic and cannot multiply outside a host. Indeed, outside a host cell, they are virtually lifeless. When a virus enters a host cell, it takes over the host's chemical energy and protein-making ability in order to multiply itself. After the virus multiplies, the new viruses burst out of the cell, often leaving it dissolved.
All parasites are harmful to their host, but some are beneficial to humans.
Some bacteria, for instance, help keep down populations of other micro-organisms. Tapeworm Tapeworms or cestodes live in the intestine. They usually come from eating uncooked meat or fish. There is such an enormous range of different microbes that scientists have found it hard to decide how to group them. Microscopic algae and mould are sometimes thought of as kinds of plants, or as part of a separate group called protoctista .