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In 421 BCE Hippocrates (466-377 BCE) recorded the first description of influenza. To understand why people get the flu so often you need to learn a little about the viruses that cause the flu. Scientists have identified 3 types of flu viruses A,B,C.
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Type C is the least serious type of influenza. People with type C have mild symptoms and seldom die. Type C doesn't mutate, and like chicken pox you develop an immunity to it. Immunity means your white blood cells have learned how to destroy the virus so you can't get it again.
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Type A and B are more serious. In 1918 20 million people died of "Spanish flu," or A (H5N1). The H and N stand for proteins on the virus' coat. A virus is made of a protein coat or box with either RNA or DNA inside the coat. H is hemagglutinin and N is neuraminidase. Scientists use the 5 and 1 to indicate which mutated versions of these proteins the virus has. Type A and B mutate and change the proteins in their coats so your immune system can't recognize them. Each time you catch the flu it has a different protein coat so your immune system must make new antibodies to fight it.
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Birds can also get type A and B flu. Scientists think that many of the new mutated flu strains come from birds. As birds migrate around the world they spread the flu virus to other birds. The virus mutates in a few of the millions of sick birds. The mutated virus then jumps from the bird population to people and we have an out break of flu. Pigs and horses can also get the flu. Some scientists think the "Spanish flu" came from pigs in the US.
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This is an electron microscope image of a flu virus.
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The symptoms of flu are: fever, cough, sore throat, runny and stuffy nose, headache, muscle aches, and fatigue. A secondary bacterial infection can develop if the person is old or has a weak immune system that is compromised. Older people sometimes develop pneumonia after they get the flu. Pneumonia is caused by bacteria in the lungs. Pneumonia is more serious. If it isn't treated early enough with antibiotics pneumonia can be fatal. Flu is a virus, not a bacterium, so antibiotics can't help.
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Scientists develop vaccines to protect people from the flu. The vaccines are made of dead flu viruses. Your body learns how to make antibodies against these dead viruses. Later if you get exposed to live flu viruses your body already knows how to attack them so you don't get sick. Every time the virus mutates the scientist have to make a new vaccine.
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During the winter of 1967-68 the Hong Kong flu, A (H3N2), struck the US. The scientists didn't have a vaccine for this new strain and 34,000 people died.
