This is a work in progress... please bear with me, come back and watch it grow? Send constructive suggestions?
Viruses are so tiny that they weren't seen before the invention of the electron microscope in the 1930s.
Certain scientists had proved that there must be some very tiny organism before that, but very little was known about these, and nothing was known or even suspected of many other viruses.
Indeed, it was not so long ago that people disputed whether a virus was actually a living thing, rather than something more akin to a poison.
If you recall the Seven Great Challenges of Life, then as you study viruses, you will see that they succeed in overcoming all of those challenges. Organisms in the virus kingdom are generally seen as living things today, although some biologists say that they "just barely" qualify.
The slight problem is that viruses can only reproduce by "hi-jacking" the mechanisms in the cell of another organism. But if you aren't going to "allow" that, then why do you count, say, a cow as living? It can't live without grass to eat, after all. Viruses also "get around" having to deal with some of the other challenges by letting the cell they are infecting take care of those things for them. But they do reproduce, even if it is by an exceptional mechanism. For this, they get my "vote" as being "alive".
Viruses cause many horrific diseases... and the fact that they are "just barely" living makes them harder to kill. Antibiotics don't kill viruses, for instance. JHappily, there are vaccines for many diseases caused by viruses, and the academics and pharmaceutical industry are increasingly successful in creating anti-viral drugs.
They also cause 'flu. It can, of course be extremely serious, particularly for the elderly or very young. But for many, many people every year, it is merely nasty. But nasty for many, many people.
While we notice a virus which makes us ill, we sometimes fail to notice unsung virus heroes. All of life is interconnected. When we took away all of the wolves in Yellowstone Park, the elk populations grew so much that they ate their food plants so extensively that starvation became a possibility.
Given that it is hard for us to see viruses, we probably still have a great deal to learn about the good things they do for us without us even realizing. Many viruses "eat" bacteria, almost as the wolves ate the elk. Without those viruses, we might find we had a problem with the bacteria becoming too abundant.
Scientists' best guess at the basic physical design of some viruses... supported by fuzzy electron microscope images... show objects of remarkable beauty in some cases. A virus can look like a lunar lander, or an elegant geometrical solid.
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