The adult stage of the honey bee is also affected by diseases, but both symptoms and damage done to colonies are often less well defined than those resulting from brood diseases. This does not mean, however, they can be ignored. The most damaging adult disease appears to be nosema, caused by a microsporidian that infects the digestive system. The incidence of nosema very often is correlated with stress on a colony. Several viruses also affect both adult worker and queen honey bees, and certain worm-like parasites called spiroplasmas found in nectar have been shown to be deleterious to workers.