"Last week we talked about vaccines," Don told his students. "This week, we're going to talk about why they work, and what causes most forms of disease, which is germs. For centuries, the prevailing theory, in the West, at least, was that miasma, or something bad in the air, caused disease, although in different times and places you saw people starting to see the holes in that theory via observation and logic--noticing how disease could pass directly from person to person, usually. Or maybe they would figure out that hey, wait a minute, it's the water that's causing this particular disease, but they still didn't know why. That's because the human eye can't see viruses or bacteria, aka 'germs.' They're too tiny.

"In 1674 a Dutchman named Anton van Leeuwenhoek designed and built his own microscope, a device capable of magnifying our vision of something by many, many times, and observed the first single-celled organisms in a drop of water from a pond. Over the next couple of centuries, scientists learned more and more about microbiology, and by the late eighteen hundreds what they had learned solidified into germ theory: transmissible diseases are caused by 'germs,' these microscopic organisms that invade the human body and have to be fought off by the immune system. Most germs are either bacteria or viruses, and for practical purposes what you need to know is that you can fight a bacterial infection with antibiotics, but not a viral one.

"This week we have a microscope with a slide featuring a droplet of pond water, just like the one Anton van Leeuwenhoek would have observed back in the day." Although this one was from Fandom's pond, so Don had taken a quick peek through the viewfinder before class to make sure there wasn't anything really weird in there. "I'd like everyone to come up and take a look, and see just how much is going on under the surface."