Varroa exists

There is one threat every colony you keep will face, everywhere you keep it: Varroa destructor, a mite the size of a pinhead that feeds on developing bees and spreads viruses while it does. It is universal wherever honey bees are kept, and the single largest cause of colony loss anywhere it has taken hold.

Left unchecked, its population does not creep, it compounds: roughly 2.4 percent growth a day while there is capped brood to breed in works out to doubling about once a month, so a small spring infestation becomes an overwhelming autumn one with no dramatic symptom in between.

You cannot see this coming by looking at the entrance.