Phages are 15 times more likely to find a victim if they stick to mucus. And since mucus is universal in animals, and phages are universal in mucus, this partnership probably started at the dawn of the animal kingdom. In fact, Rohwer suspects that phages were the original immune system – the means through which the simplest animals controlled the microbes at their door.24 These viruses were already plentiful in the environment. It was a simple matter of concentrating them by giving them a layer of mucus in which to anchor themselves.
