Among all infectious diseases affecting horses, respiratory disease pose the greatest threat to horses kept in stables, horses used for breeding and race horses. Here a distinction should be made between the so-called monocausal infectious diseases (so-called Henle-Koch postulates) and multicausal infectious diseases which are the result of the synergistic interaction of different processes, that alone do not lead to disease. There is no clearcut distinction between the two groups. The most important monocausal respiratory infections of horses are caused by equine influenza virus (subtypes 1 and 2), equine rhinopneumonitis virus (equine herpes-virus type 1), equine arteritis virus and partially by Reoviruses. In addition, streptococcus equi (strangles, adenitis equorum, coryza contagiosa equorum) and mycobacteria tuberculosis can cause monocausal diseases. In multicausal infections, the first step usually is a virus infection. This is the basis for secondary infection by widespread, opportunistic agents such as bacteria, mycoplasms or fungi which lead to clinical disease. The method of choice for controlling monocausal respiratory infections of horses is prophylactic vaccination and chemotherapy. Measures to control multicausal infections include: vaccination with functional-synergistic combined vaccines; the use of herd-specific vaccines; medical stimulation of the non-specific part of immunity (immunmodulation, paramunization). Paramunization is a new concept in the prophylaxis and therapy of respiratory infections of horses and can be combined with prophylactic vaccination as well as with chemotherapy. In severe cases of respiratory disease paramunization can also be combined with corticosteroids.