Outer limits

The most pervasive type of organism on the planet is a type of virus called a bacteriophage. There are hundreds of millions of them in your mouth, lungs and on the film of your eyes. Lots of them look like tiny spiders. They’re not interested in your cells, though: they merely infect the bacteria that also teem in your body, hijacking their molecular machinery to generate more copies of themselves.

For the past three billion years – give or take – bacteria and phages have been locked in an evolutionary arms race. One of the bacterial defence mechanisms involves cutting an invading virus into pieces,