@millimat My dad is in hospital at the moment attempting to recover from an antibiotic-resistant infection. He's been there since we took him to A&E on Boxing Day with symptoms of sepsis. He has been given high doses of antibiotics intravenously every 4 hours every day since then, but they are not working because the bacteria which has taken hold is resistant to antibiotics. Eventually he has been seen by a specialist in infectious disease medicine who has prescribed the one antibiotic left that is still working against the most resistant of bugs. They protect this particular antibiotic from overuse because they are terrified bacterial infections will become resistant to it too, so it's use is the last resort and closely monitored.
How do bacteria become antibiotic resistant? Well, people take them incorrectly and allow the bacteria to build up defences against them. If, say, you have an infection that needs a 7 day course of antibiotics, the doctor will prescribe a 7 day course and that'w what you should take. But many people start to feel better before the 7 days is up and stop taking them. The first few days kills off the bug that is making you feel unwell and allows your body to begin to feel better, but if you stop the course early you may leave behind some stronger bugs which are not currently making you feel unwell but could do if you were run down. These stronger bugs have been exposed to the antibiotic and had time to work out how to survive against it. They reproduce and fill the void left by the weaker bugs that were giving you symptoms. These bugs that have learnt to survive include MRSA which lives on many people without causing symptoms. But when a person with a surgical wound or who has a weak immune system is exposed to it they are unable to fight it off using their own immune system and the antibiotics no longer work either.
The numbers of bacteria that are learning to survive against antibiotics is on the increase and by taking half a course here and half a course there against illnesses that aren't even bacterial is just giving the more dangerous bacteria more chances to build up their defences. Antibiotics should only be taken where there is a clear need for them and once prescribed they should be taken to the end of the course. If you have half a course in your cupboard it means that not only have you given the bacteria a first chance at developing resistance by not completing the course, if you take the other half at a different time you are giving the bacteria a second go.
Many people are not aware that casual use of antibiotics can cause such a massive problem, but in terms of things that could severely curtail human population numbers antibiotics resistance is up there as a biggie alongside global warming. I certainly do not want to return to a time pre-penicillin where a simple wound to the skin could result in death and childbirth is as perilous as it was back then.