There are many things that kill viruses, like fire, for example. Even if I thought that a candle in the incubator could be used to kill viruses, there are additional reasons why I should not put a candle in the incubator. Some of these I could anticipate (heat, oxygen consumption) and others I could not (does soot affect egg viability...if a candle is safe to use near humans, is it also safe for developing chicks?). In other words, being the cautious sciency-guy that I am, I would think about the possible problems that could arise from using a candle, try to realize my assumptions and also recognize that there are factors that cannot be easily anticipated. If I wanted to be double-sure that a candle were safe to use, I would make sure that I didn't have all my eggs in one incubator the first time I tried it. In other words, I would go slow and experiment first so as not to risk a whole batch of eggs (aka trial and error).