The first pandemic of the 21st century:2009 A (H1N1) influenza

In early April 2009,a novel influenza A(H1N1) virus emerged in Mexico and the USA.The virus spread worldwide quickly through human-to-human transmission.This virus contains gene fragments from human,avian,and swine influenza viruses.The HA gene of this virus was highly homologous to the HA gene of 1918 pandemic influenza strain.The virus appeared to primarily affect children,young adults,pregnant women,as well as those with underlying cardiac or lung disease condition.Its transmissible ability among humans appears to be higher than that of seasonal influenza.Some infected patients have vomiting and diarrhea,which are rare in seasonal influenza.Previous influenza pandemics and 2009 influenza A(H1N1) virus pandemic show that,as the influenza virus mutates fast and prone to gene reassortment,the new variant strain is likely to cause a new pandemic and pose a threat to human health.Since swine are known to be permissive to both avian and human influenza viruses,they have been proposed as a mixing vessel for the generation of new pandemic viruses through gene reassortment.