So, when humans use antibiotics to kill off bacteria, in some cases, those bacteria spontaneously mutate their genes, which changes their makeup in such a way that the antibiotics cannot kill them. The bacteria that survive those encounters pass these genes on to other bacteria through simple mating (technically known as 'conjugation') – and those resistant bacteria can spread from one living thing to another.

The tricky part of this is that bacteria can share these genes with each other across bacterial species – so they don’t even have to be that genetically similar to pass along resistance. Humans and animals, who are teeming with trillions of different types of bacteria, then pass the resistant bugs along to each other. And, on top of it all, we introduce those resistant species to each other inside our own bodies. So, even if a human or an animal has been exposed to an antibiotic just once in their lives they can contain mutant bacteria that can be easily spread.

Bacteria, it turns out, don’t care about political borders or immigration policies – for example, researchers have even found drug-resistant bacteria on the rear-ends of seagulls in Lithuania and Argentina.

The most important part of this is that bacterial resistance is essentially a numbers game: the more humans try to kill bacteria with antibiotics, and the more different antibiotics they use, the more opportunities bacteria have to develop new genes to resist those antibiotics. The less we use, the less bacteria can develop and share resistance.

How big is the problem?

It’s hard to say for sure, but the US Centers for Disease Control and Prevention (CDC) estimates that in the US alone there are about 23,000 people who die every year from antibiotic-resistant infections. For example, they estimate that resistance to antibiotics that treat Clostridium difficile (C. difficile) causes almost 500,000 infections in the US every year, which lead to about 15,000 deaths. (But Amanda Jezek, a spokesperson specialising in policy and government relations at the Infectious Diseases Society of America, a group that represents many of the country’s infectious disease doctors and scientists, says the overall number of deaths is a conservative estimate and likely higher.

Meanwhile, a 2015 study published in Nature found that global antibiotic consumption went up 30% between 2000 and 2010.