bacteria
/ bak-TEER-ee-uh /
Bacteria are tiny living things, each made of a single cell so small that millions could sit on the head of a pin. They are everywhere — in the soil, in the sea, on your skin, and by the trillions inside your gut right now. A single drop of pond water can hold more of them than there are people on Earth.
Here is the surprise: the vast majority are harmless, and many are downright helpful. Bacteria in your gut help digest your food, and others turn milk into yogurt and cheese. Only a small minority are troublemakers — the kinds that cause strep throat, tuberculosis, or food poisoning.
For most of history nobody knew germs caused disease. Then, in the 1880s, Robert Koch laid down a clear test — now called Koch's postulates — for proving that a specific microbe causes a specific illness: find the germ in every sick patient, grow it in pure culture isolated from the body, use it to make a healthy host sick, then recover the same germ again. That chain of proof turned vague suspicion into hard science and helped found modern medicine.
Bacteria that fall on a warm, moist surface can split in two roughly every 20 minutes — one cell becomes millions overnight.
Why food left out at room temperature spoils so fast.
The word comes from the Greek bakterion, meaning "little staff," because the first ones seen under a microscope looked like tiny rods. The singular is "bacterium"; "bacteria" is plural. They are not the same as viruses — bacteria are full living cells, while a virus is a far smaller scrap of genetic material that cannot live on its own. This matters in practice: antibiotics fight bacteria but do nothing against viruses like the common cold.