Are prokaryotic cells beneficial or harmful to humans? 🔬

Good: They live in our gut and make vitamin K, as well as the "beneficial" bacteria etc outcompeting the more dangerous ones. They help with other aspects of digestion, though sometimes that may make us a little gassy as they ferment small quantities of undigested or partially-digested food.

A large portion of the oxygen we breathe comes from prokaryotes in the ocean completing photosynthesis, using the CO2 dissolved in the water, and making more "space" (preventing complete saturation of the oceans with carbon dioxide) for CO2 from the atmosphere to be absorbed by the ocean.

Bad: There are a wide variety of diseases caused by prokaryotes...tuberculosis, bacterial meningitis, Legionnaire's disease, acne...