The mites belong to the genus Parasitellus and this individual is particularly heavily infested, but her plight isn't as dire as it might seem. These mites don't feed on the bee or transmit disease in the way that Varroa mites of honeybees do. They're commensals, living in the bees' nests and eating the sticky coating from pollen grains, as well as consuming debris that accumulates in the nest. Other than using some of the pollen that the bee collects, they don't seem to do any serious harm and may indeed be beneficial.
It's usually newly-emerged queens that are as heavily infested as this and it does sometimes seem that they are struggling under the load of hitch-hikers, but after a bit of a rest this one took to the air again without much difficulty. The mites tend to congregate in parts of the bee where it's difficult for their host to comb them off with its legs.
The mites spread between bumblebee nests via flowers. When as infested bee visits a flower a few hitch-hikers dismount and hide in the blossom until another pollinating bee arrives, then they climb on board and are carried back to its nest.