The current Eurosurveillance is all about influenza A viruses, in particular their transmissibility and evolution. Here's an excerpt from the long editorial by Herfst and Fouchier on Epidemiological and genetic investigations of human-to-human transmission of zoonotic influenza viruses:
Of the over 1,400 species of infectious organisms known to be pathogenic to humans, 60% are zoonotic, i.e. transmissible from animals to humans . We therefore continuously face the threat of newly emerging pathogens with a major health impact. In the last decade, viral zoonoses have resulted in numerous human cases of infection with e.g. influenza A viruses, Ebola virus, Nipah virus, Hendra virus, severe acute respiratory syndrome (SARS) coronavirus, and Middle-East respiratory syndrome (MERS) coronavirus.
While these zoonotic viruses often caused severe disease and deaths in humans, they generally did not represent global health threats as the viruses lacked the ability of sustained human-to-human transmission. However, some of these viruses might adapt to replicate in and spread between humans, to cause pandemics. This is true in particular for respiratory viruses associated with recent zoonotic outbreaks that belong to families that also contain viruses that are endemic in humans and thus have been shown to possess pandemic potential in the past. ...
Monitoring and predicting which of the various zoonotic viruses have the potential to cross the species barrier and emerge in humans globally has become a major topic in infectious disease research.
Wildlife can host an enormous diversity of viruses, which generally do not cause severe disease in these reservoir hosts. Bats have been recognised as reservoir hosts of viruses of the Filoviridae, Paramyxoviridae, and Coronaviridae families [2,3], while wild birds harbour the greatest diversity of influenza A viruses.
Recently, close relatives of avian influenza A virus were also found in bats . Occasionally, viruses are transmitted from their reservoir hosts to other animal species, intermediate hosts, from which the viruses may subsequently be more efficiently transmitted to humans, e.g. as a consequence of more frequent contact.
Unfortunately, very little is known about the genetic and phenotypic viral and host traits that facilitate interspecies transmission. In addition, knowledge on the mechanisms by which these zoonotic viruses may subsequently adapt to efficient replication and spread in humans is lacking. Recently, progress has been made towards understanding the genetic and phenotypic requirements of avian influenza viruses to become transmissible in ferrets [5-9].