I think that Bill Bryson, in A Short History of Nearly Everything, explains bacteria best:
Because we humans are big and clever enough to produce and utilize antibiotics and disinfectants, it is easy to convince ourselves that we have banished bacteria to the fringes of existence. Don't you believe it. Bacteria may not build cities or have interesting social lives, but they will be here when the Sun explodes. This is their planet, and we are on it only because they allow us to be.
Bacteria, never forget, got along for billions of years without us. We couldn't surive a day without them. They process our wastes and make them usable again; without their diligent munching nothing would rot. They purify our water and keep our soils productive. Bacteria sythesize vitamins in our gut, convert the things we eat into useful sugars and plysaccharides, and got to war on alien microbes that slip down our gullet.
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So why, you are bound to ask at some point in your life, do microbes so often want to hurt us? What possible satisfaction could there be to a micbrobe in having us grow feverish or chilled, or disfigured with sores, or above all expire? A dead host, after all, is hardly going to provide long-term hospitality.
To begin with, it is worth remembering that most microorganisms are neutral or even beneficial to human well-being. The most rampantly infectious organism on Earth, a bacterium called Wolbachia, doesn't hurt humans at all -- or, come to that, any other vertebrates -- but if you are a shrimp or worm or fruit fly, it can make you wish you had never been born. Altogether, only about one microbe in a thousand is a pathogen for humans according to National Geographic -- though, knowing what some of them can do, we could be forgiven for thinking that that is quite enough. Even the mostly benign, microbes are still the number three killer in the Western world, and even many less lethal ones of couse make us deeply rue their existence.
Making a host unwell has certain benefits for the microbe. The symptoms of an illness often help to spread the disease. Vomiting, sneezing, and diarrhea are excellent methods of getting out of one host and into position for another...
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Precisely the same thing happens with meningitis. At least 10 percent of young adults, and perhaps 30 percent of teenagers, carry the deadly meningococcal bacterium, but it lives quite harmlessly in the throat. Just occasionally -- in about one young person in a hundred thousand -- it goes into the bloodstream and makes them very ill indeed. In the worst cases death can come in twelve hours. That's shockingly quick. You can have a person who's in perfect health at breakfast and dead by evening."
Bacteria are fascinating things, indeed.
