SO ENDS this first of two weekends on the cunning strategy of the bacteria Wolbachia which has enabled it to colonize half of the insect species in the world. The ability of symbionts and parasites to enact sophisticated strategies on their hosts makes one suspect that more than chance is at play, but when nature gambles at such high rates over geological time, even slim probabilities can become realities. It is a game of sheer numbers against the odds of chance.