What generally affects only worker larvae in a colony?

Chalkbrood is a fungal disease that specifically impacts the larvae of honey bees, particularly worker larvae. The disease is caused by the fungus Ascosphaera apis, which infects the larvae when they are young and still in the brood stage. In a colony that is affected by chalkbrood, the infected larvae typically become mummified and can be observed as small, chalky white to gray balls in the brood frames.

In contrast, other ailments mentioned in the list have broader or different effects. American Foulbrood (AFB), for instance, can affect both worker and drone larvae and is caused by the bacteria Paenibacillus larvae. Viral infections can impact various life stages and are not limited to just the worker larvae; they can also affect adult bees. Nosemosis, primarily caused by the Nosema parasite, generally affects adult bees rather than young larvae.

Therefore, chalkbrood is the only condition among those listed that specifically targets only worker larvae, making it the correct answer.