The tracheal mite
Tracheal mites were first discovered in the U.S. in 1984. They are barely visible to the naked eye and are rarely seen by the beekeeper (Wikipedia says they are too small to see, but I have seen a few. They are visible if you have good eyesight). They attack honeybees by living and breeding in their tracheal tubes, essentially suffocating them.
Although these pests have been blamed for countless colony collapses during the past few decades, we hear about them less and less these days. This is most likely because many bees have developed hygienic grooming behavior which helps to prevent heavy infestations.
Many people may not realize that natural-size bees are resistant to tracheal mites because the tracheal tube in a worker bee raised in 4.9mm cells is typically too narrow for a tracheal mite to enter.
Obviously, regressing your bee colonies or starting out with regressed bees are the excellent ways to fight tracheal mites.