Novel and Variant Influenza Viruses
Influenza (flu) is a respiratory illness caused by influenza virus that can infect both humans and animals. Human Influenza type A and B viruses commonly spread in people and are responsible for seasonal flu cases and seasonal epidemics in humans each year.
There are other Influenza type A viruses that circulate in wild and domestic birds, swine (pigs), horses, dogs, and bats that do not commonly cause illness in humans but do cause sporadic cases each year. These typically occur in humans with close contact with sick animals. These viruses do not typically pass well between humans, most people who become sick with a novel or variant flu virus from animals will have similar symptoms and recovery to seasonal flu and will not pass the virus to any other humans.
Novel and variant flu viruses are of public health concern primarily because they have the potential to mutate and become easily passed between humans. This could happen if an animal or a human is co-infected with more than one type of flu at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus. This type of major change in the influenza A viruses is known as antigenic shift. If this new virus causes illness in people and can be transmitted easily from person-to-person, an influenza pandemic can occur. This is what happened in 2009 when an influenza A H1N1 virus with swine, avian and human genes emerged in the spring of 2009 and caused the first pandemic in more than 40 years.
Avian Influenza H5N1 Virus (Bird Flu)