There are many kinds of viruses living in some living cells, and they directly use the substances in the cells as their own nutrition for parasitic life. Such as hepatitis virus, HIV, tobacco mosaic virus, etc.
Some bacteria live in humans, animals and plants, and are also parasites, such as Escherichia coli and Shigella dysenteriae. There are also some unicellular protozoa parasitic life. Such as amebic dysentery and plasmodium. Ascaris lumbricoides is a multicellular animal and a common parasite in human intestine, which directly absorbs nutrients digested by human body as its own nutrition. In addition, there are tapeworms, pinworms, schistosomiasis, etc., all of which are parasites harmful to human body. Body lice and fleas living on the surface of human body are common body surface parasites, and they mainly feed on human blood as their own nutrition. Some bacteria that make people suffer from tinea pedis live between toes and soles of feet and rely on absorbing body fluids as their own nutrition.
The genes of some parasitic microorganisms can be integrated into the chromosome of the host and become the genetic diseases of the host.