Prions
I went to the doctor yesterday and learned about prions. The internet says they are small, misfolded proteins, that they are not alive but can get other proteins to similarly misfold. I read they are too small to see individually but that a clump of them can be seen with a good electron microscope. The doctor, a dermatologist, was remarking in response to my statement that I understood that the existence of viruses had only been known since the end of the 1800's, mostly because they are so small. The doctor said that prions are much smaller and were only known since the 1980's.
All of this was incidental to the visit, which was to have a bubble-like growth on the tip of my ear and a crusty spot on my forehead removed.