A mite the size of a pinhead

Varroa destructor is barely bigger than the head of a pin, but it is the single largest driver of colony loss anywhere honey bees are kept. It is an external parasite, riding on adult bees and feeding on the fat body of both adults and developing brood, the tissue that stores a bee's energy reserves and supports her immune system.

For years it was believed to feed on blood instead; it does not, and the correction matters because fat-body damage explains why infested bees emerge weakened rather than simply thinner.

While it feeds, the mite injects RNA viruses directly into its host, driving viral loads far higher than infection through normal contact ever would. A mite you can barely see is, in practice, the deciding factor in whether most colonies you manage survive their first winter.