Randy writes: I strongly suspect, as does Jerry, that nosema is far more of a problem if the bees are already immunologically stressed by chilling, poor nutrition, and viruses.  

Re: chilled brood, Bailey & Ball state

Larvae of all ages sometimes die without any apparent infection. ''Chilled brood" is then often diagnosed, the implication being that not enough adult bees were present to cover the brood and keep it warm when they clustered at night or in inclement weather. However, unsealed larvae can survive several days at room temperature (about l 8°C) without food, so severe or prolonged chilling would be necessary to kill them. 

However, A. apis [chalkbrood] grows best in slightly chilled larvae as its optimal temperature for growth and formation of fruiting bodies is about 30°C (Maurizio, 1934). Experiments have shown that brood is most susceptible when chilled immediately after it has been capped (Bailey, 1967b). The chilling need be only a slight reduction of temperature, from the normal 35°C to about 30°C, for a few hours; and it can easily occur, even in warm climates, in colonies that temporarily have insufficient adult bees to incubate their brood adequately. Larvae are most likely to be chilled in early summer when colonies are growing, and drone larvae often suffer most as they are generally on the periphery of brood nests.   

The smallest colonies are at the greatest risk of becoming chilled because they have the lowest capacity for heat and relatively large surface areas. Heath (1982a, b), in extensive reviews, quotes several observations that chalk-brood is aggravated when colonies are rapidly expanding in spring, i.e. when the ratio of brood to adult bees is high, or when it is increased experimentally; and that very small colonies used for mating virgin queens or in observation hives are very susceptible. 

Koenig et al. (1987) also noted that decreasing the ratio of adult bees to brood aggravated chalk-brood; and Pederson (1976) showed that artificially heating hives in spring diminished the incidence of the disease. Other non-lethal factors, such as slight infections by viruses or bacteria, or poisoning, or inadequate food from diseased nurse bees may well cause the same effect as chilling by slowing the rate of development of larvae.   

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