Colorized H5N1 influenza virus, also known as avian flu. Source
Bird flu, also called avian flu, is caused by an influenza virus that normally only infects birds, though it is becoming more common in pigs. Avian flu is a strain of type A influenza known as H5N1, different from the swine flu of the recent outbreak (which is H1N1).
Though the outbreak of influenza among birds in recent years has been extensive and resulted in the culling or death of many thousands of birds, the number of human cases has been minimal and human to human transmission rare. According to the World Health Organization, as of April 2009, approximately 421 humans have been infected with the virus and there have been roughly 257 deaths (these are cumulative numbers since the outbreak began). Most infections have occurred in areas reliant on poultry and those where the poor or the farmers live in close contact with wild and domestic fowl, including Indonesia and Vietnam. Mortality has been highest in those aged 10-19 years, according to the U.S. Centers for Disease Control and Prevention (CDC).
The first human cases were documented in the open farm markets of Vietnam, China, and Indonesia in 2005, though the animal outbreak has been ongoing since 2003. In 2006, the first case was reported in the Middle East. Since that time, Egypt has had the fourth highest number of deaths (23) and the third highest number of infections (67). There are a few widely sensationalized cases of spread among family members, such as in Thailand in 2004 and Indonesia in 2006 (killed 7 of 8 infected family members), but the spread was not beyond one person in each case (contact with dead fowl is the most common factor of family infections). The H5N1 strain of bird flu, as of now, has shown a lack of ability to bind to the cells of the human respiratory tract, which is necessary for the virus to infect humans and limits its spread. Mutations in the virus's ability to adhere to cells in the human respiratory system would make it a formidable opponent for modern medicine.
A Chinese father who contracted bird flu from his son was found to have been in contact with 91 people, none of which contracted the disease. Genetic analysis (published by Lancet in 2008) found that the bird flu strain had not undergone any major mutation that would allow it to spread more readily among humans. The World Health Organization (WHO) has been quoted as saying that the origin of human infection with bird flu is via bird droppings. Ingestion may be introducing a new route of transmission for this strain as opposed to the familiar aerosol, or airborne, transmission that influenza most commonly uses. Fecal transfer of disease is common for many pathogens.
So far, the evidence seems to indicate a genetic susceptibility to H5N1 infection and alternate transmission routes before human infection can occur. This is good news since the virus is extremely lethal to those who contract it. However, if the rate of mutation seen in the common flu virus is any indication, bird flu will find a way to more directly harm humans in the near future. Some governments are stockpiling antiviral medications, such as Tamiflu, for the possibility that a human pandemic is coming (and has been helpful for other outbreaks, such as H1N1). Research has found that suppressing viral replication is key to treating any human infections that may occur because the high mortality rate (60%) is due to highly efficient replication and corresponding increased host immune response. Also, the flu vaccine is being tested and adjusted to possibly cover the H5N1 strain, which it currently does not do.