The spread of West Nile virus (WNV) in North America since 1999 has reawakened concern about this pathogen in Europe. WNV can cause West Nile fever in humans, and in a small proportion (around 1 in 150) the disease can take a severe course associated with symptoms of the central nervous system (encephalitis) and even death, particularly in older patients (>70 years). In contrast to the USA, where the virus has spread from New York thronghout the continent to the west coast, only temporally and regionally limited outbreaks of WNV infections have been observed in Europe since the 1950s. Birds serve as the reservoir for WNV and the transmission of the virus occurs predominantly via mosquitoes. Ornithophilic mosquitoes transmit the virus amongst the bird population while those mosquito species that feed on both birds and mammals can transmit the pathogen also to humans. However, mammals are considered to be blind alleys that do not contribute to the epidemic spread of the pathogen. The differences so far observed between the epidemics in North America and in Europe might be explained by the following considerations: Europe has long had contact with WNV-endemic areas in Africa via migratory birds, whereas the pathogen was first imported into the USA in 1999 where it has infected a "naive" bird population. It is possible that the strain presently spreading through the USA is highly pathogenic, whereas strains of varying pathogenic potential circulate in Africa. This could resulut in a natural immunization of the bird population through contact to strains of low pathogenicity. The possibility of a natural resistance in European birds is also being considered since these animals have been confronted with the pathogen for long periods of time. Investigations into the prevalence and incidence of WNV infections in German bird populations as well as in dead end hosts such as humans and horses should provide information regarding the potential risk represented by WNV.