An overview of the most important infections which can be transmitted from humans to pet dogs and cats is presented. Two quite different sources of infection stand diametrically opposite each other: 1. The transmission of active human infections to dogs and cats and 2. the transmission of infectious agents by feeding raw meat, offal, unsterilized milk products, kitchen scraps and contaminated feedstuffs. Humans can be the source of the following infections: 1. Zoonoses with reciprocal modes of transmission, e.g. Campylobacter and E. coli infections, trichophyton and microsporum infections, reo-, parainfluenza-, adeno, rota- and corona infections. 2. Zoonoses in which the main direction of infection is human----animal, e.g. tuberculosis and influenza A. 3. Infections originally pathogenic to humans which meet an impasse in dogs and cats (blind alley hosts), e.g. herpes simplex, varicella-zoster, measles and Corynebacterium diphtheriae. Listeria, salmonella, campylobacteria, toxoplasma, fungi, yeasts and viruses are transmitted via feed. The most dangerous virus infection to be transmitted to cats and dogs via raw pork leftovers is Aujeszky's disease. The dog or cat, which is the last link in the infection chain, suffers an agonizing death. The other infections originating from feed must be assessed quite differently. They are links in infection chains, which spread pathogens and endanger the health of man and animal in turn. A typical example is toxoplasmosis. Man becomes infected via sporulated oocysts from feces. Pet cats mainly become infected via raw pork containing cysts.