they are only genetic material and a few proteins that form a hollow ball and a tail. They rely on the metabolic energy of their bacterial hosts to reproduce and spread to new hosts. But they are not simple. Despite their host specificity, phages are not picky. They will squeeze almost any genetic material into their tiny heads whether it is helpful for their life cycle or not, thus changing the nature of their next host, and their interaction with it. Combine this promiscuous movement of genes with their mind-blowing numbers, and bacteriophages are capable of forming and collapsing entire ecosystems.