The viruses then may thanklessly destroy their benefactor cell upon exiting. The list of diseases caused by viruses is long and frightening. It is highly improbable that anyone reading this book has entirely avoided viral diseases or will in the future. As of this writing, it is a fair generalization that our technology has failed to produce a cure for viral diseases-but it will. An irony in which one might derive some satisfaction is that viruses will help us, as a result of genetic engineering, develop a cure for viral diseases and to make many other contributions to the science of medicine.
Bacteria represent the oldest known group of organisms.
They are responsible for much of earth's atmospheric oxygen, in prehistoric time and yet today. They have altered soils through nitrogen fixation and mineral utilization. They have been a source of organelles and photosynthetic pigments through endosymbiosis. Bacteria make a high cellulose diet feasible for cattle, reduce the world's litter through decomposition and manufacture methane fuel. They are used in the manufacture of cheeses and antibiotics.