In the last two to three decades a significant increase of viral zoonotic 
infections was observed. These zoonoses are not only newly (or previously 
unrecognized) emerging diseases, but also due to the reappearance of diseases 
thought to have been defeated (re-emerging diseases). "New" viral diseases can 
arise when viruses broaden their host-range (monkey poxvirus; equine 
morbillivirus), or can be a consequence of intrinsic properties of the virus 
itself, such as high mutation rates (influenza A virus). Most new or reemerging 
viral zoonoses are due to infections with hemorrhagic viruses. Many of them are 
transmitted by insects (arboviruses, e.g. yellow fever virus) or by rodents 
(e.g. Hanta viruses), others by contact with patients and nosocomial infections 
(e.g. Ebola virus). The emergence and increase of these diseases are a 
consequence of anthropogenic environmental changes, such as distortions of the 
ecological balance and changes in agriculture. In addition, the uncontrolled 
growth of the cities in tropical and subtropical regions without improvement of 
the public health measures and the increasing international animal trade and 
travel also favour the spread and recurrence of these diseases.