Viruses mutate -- frequently. Then the mutations mutate, etc., etc., etc. That's how virus-based epidemics and pandemics develop. They jump from species to species until a mutation allows them to spread within the species. Example: bird flu has crossed into cows. Cows at this point might not be getting it directly from exposure to each other, but from contaminated equipment being used from cow to cow. People who work with cows get it. So far, it is not spreading from person to person, but that's likely to change after the virus mutates often enough. Once that happens, there will be regional epidemics first, then a world wide pandemic.
People shrug it off when it's "only the flu." But the 1918 flu pandemic (aka Spanish flu) killed millions around the world, most of them young people, who seemed to be the most vulnerable.
In 1957-58 there was a world wide pandemic known as Asian flu. Travel was not as wide spread and frequent then as it is today so it took a while to spread. It reached the US in the late spring of 1957 and seemed to diminish by summer. But, in the fall, when flu season started up again, Asian flu came back with a powerful wham. I was in grade school then (3d grade). My 3 siblings and I all had it at the same time. They recovered within a couple days. I was sick for a week. On the third day, my temp rose to 105.6. Scared my mother so much that she dropped the thermometer, spilling mercury onto the floor when it broke. The doctor ordered aspirin and an ice bath, which brought the temp down gradually, but I developed a kidney infection and needed antibiotics and fluids for it.
Estimated deaths world wide were between 1 to 4 million. A recession followed that pandemic so that the plant where my father worked cut back employee hours to 3 days a week, sometimes only 2. He took on odd jobs to keep us afloat financially.
A couple decades later, an offshoot of the Asian flu virus caused the Hong Kong flu.
Even the flu can be dangerous.
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