If you caught pneumonia in the pre-antibiotic era, you had only a 50% chance of surviving the infection. So it is no wonder that England's chief medical officer, Professor Dame Sally Davies, is anxious that antibiotics don't lose their effectiveness. Last week, she warned that bacteria are becoming increasingly resistant to antibiotics. If this carries on, we will no longer have cures for common infections. Each year, about 25,000 people in Europe die from antibiotic-resistant infections.

But this isn't new. Since 1961 Staphylococcus aureus has been resistant to penicillin and has caused serious illness (pneumonia and skin sepsis). Methicillin-resistant Staphylococcus aureus (MRSA) is a real problem in hospitals and there have been community outbreaks too. Different antibiotics kill bacteria in various ways. They may destroy their cell wall, stop their protein production, or prevent them from copying their DNA, so they can't reproduce. Bacteria can evade destruction through random changes (and there is evidence that even before penicillin was invented some bacteria, through spontaneous mutations, were already resistant to it), or by picking up new genetic material from other bacteria. These resistant bacteria learn to inactivate antibiotics or to continue to grow despite them.

Resistance is fuelled by many things including overuse by vets and doctors. Overprescribing is a major factor – by being constantly exposed to antibiotics, the smartest bacteria learn how to avoid being killed – only take these drugs if you really need them. A third of us believe antibiotics can cure colds – they can't because colds are caused by viruses.

The solution

Most antibiotics are given by doctors for upper respiratory tract infections "just in case": the patient is unlikely to need them, but the doctor wants to play safe – in case the infection turns out to be bacterial rather than viral (tests to prove what bug causes common infections take a while and are usually not worth doing). Nor do we need antibiotics for most sore throats, coughs or earaches, and definitely not for flu.

There is evidence that not completing a course of antibiotics or reusing leftover prescriptions encourages resistance, as the drug concentrations in the bloodstream don't reach the optimal levels needed to kill the bacteria. So only the sensitive ones get wiped out and the tougher bacteria become resistant. Fewer than half of us finish an antibiotic course.

Ask your doctor how long you need to take any antibiotics for. Some antibiotics work with shorter courses (urinary tract infections are often prescribed two days of treatment), but other infections need longer courses. You should never reuse antibiotics: you won't have a sufficient dose for them to work and they may not work for your infection a second time.

You don't have to avoid antibiotics to do your bit to stem resistance – just take them as directed when you really need them and be thankful that, for now, they still work.