The "persistent" viral infections, besides "subclinical" infections, pertain to the vast group of "clinically inapparent" infections. They differ from subclinical infections by a temporally unlimited "co-existence" with the pathogen. Pathogenetically, three forms of development are possible: 1. latent infections, 2. tolerated infections, 3. occult infections. Persistent viral infections are the inexhaustible reservoir for many viruses. To the organism affected they may be of benefit (infection immunity, interference, paraimmunity) or of disadvantage (activation of the infection with conversion into a disease, cause of many chronic, slowly developing disease processes, immunopathogenic consequences), the disadvantages prevailing. To the environment, persistent infections are invisible sources of danger as they produce carriers and chronic carriers.