They enter the cell, hijack its components, make copies of themselves and burst out. Virulent phages, like many other viruses, operate on an invade-replicate-kill program.

Phages come in two main flavors: temperate and virulent. To see how this plays out, let’s take a deeper look at the phage life cycle. This means that phages have room to carry extra genetic baggage: genes that are not actually necessary for the phage’s survival that it can modify at will. In many cases, the shell has more space than the phage needs to store the DNA essential for its replication. Most phages have a rigid shell called a capsid that is filled with their genetic material. Like all viruses, phages also have high replication and mutation rates, meaning they form many variants with different characteristics each time they reproduce.

Phages are the most abundant life form on the planet, with a nonillion – that’s a 1 with 31 zeroes after it – of them floating around the world at any moment. Bacteriophage caspids can carry extra DNA that the virus can tinker with.