The National Institutes of Health reports that there are about 36,000 deaths from the flu each year. There are three different types of influenza viruses that cause disease in humans: A, B and C. According to the Arizona Department of Health Services, Type A has numerous strains that are broken down into numbered hemagglutinin and neuraminidase subtypes. Abbreviated H and N, these represent variants of proteins made by the RNA of the flu virus. For example, human strains of Type A flu include H3N2, H1N2, and H2N2.
H3N2 Flu
The seasonal flu is actually a slightly different virus almost each year, but typically breaks out in the fall and winter, often continuing into the spring. One strain of seasonal flu circulating in 2010 is H3N2. This strain causes the typical flu symptoms: runny nose, fever, cough, fatigue, and muscle aches. Symptoms can range from mild to lethal, and those who are at higher risk of serious complications from the flu should be vaccinated each year, according to the CDC, including those who have asthma, heart disease, respiratory illnesses, and AIDS, as well as those older than 65 and younger than 5. H3N2 is typically passed from individual to individual through respiratory droplets caused by coughing or sneezing.
H1N1 Flu
Flu viruses exist within many animals, such as pigs, chickens, ducks, horses and whales. According to the CDC, pigs are able to become infected with viruses from several different species, which sometimes leads to mutation, or creation of new variants of viruses that many people have no immunity to.
A new strain of flu can cause severe illness and pandemics. In 2009, H1N1, a swine flu virus, circulated and caused many illnesses and deaths around the world. It has symptoms similar to the H3N2 and is passed in the same way, although interestingly, unlike the usual seasonal strains that cause higher death rates in the elderly and young children, H1N1 was more likely to kill younger adults.
H5N1 Flu
Flu.gov reports that H5N1--also called "bird flu"--as of mid-2010 was still a serious concern for people around the world. This strain occurs in birds, but in rare cases it can infect humans. Most of the cases of people contracting H5N1 flu have occurred in individuals with direct contact with infected bird populations. H5N1 flu can be highly deadly: overall, mortality worldwide is about 60 percent.
