Image copyright SPL Image caption The race is on to find new antibiotics to treat resistant infections

Why do antibiotics stop working?

Antibiotics are designed to kill or block the growth of bacteria, but not all bacteria are susceptible. Some are naturally immune. Resistance also arises spontaneously by chance mutations. Resilient strains can then multiply and thrive - overnight, one bacterium can multiply to become a million.

When the antibiotic of choice is given it kills the sensitive bacteria but any resistant ones live on. Resistance can also be passed from one species of bacterium to another.

Is overuse of antibiotics to blame?

The more an antibiotic is used, the greater the chance there will be that bacteria will develop resistance to it. Experts are concerned that antibiotics are being used inappropriately.

Many are prescribed and used for mild infections when they don't need to be. Antibiotics cannot help you recover from infections caused by viruses, such as common colds or flu, for example.

Another issue is people who fail to take the whole course of prescribed antibiotic. Stopping treatment early means that the under-treated bacteria could turn resistant.

It is thought that the widespread use of antibiotics in animals has led to resistant strains of some bacteria being transmitted to humans through food.

Is it getting worse?

Experts are concerned that we are reaching a point where some previously manageable infections will become untreatable with antibiotics.

The superbug MRSA is now resistant to so many drugs that it is already hard to treat.

Recently, there have been reported cases of the sexually transmitted disease gonorrhoea that have been resistant to all the antibiotics normally used to manage the infection.

We are seeing cases of multi-drug resistant TB and the emergence of threatening new resistant bacteria like New Delhi metallo-beta-lactamase (NDM-1).

Resistance has now been found to the drugs used when all else has failed - such as colistin - leading some scientists to warn we are on the cusp of a "post-antibiotic era".

Resistant bacteria are spread through direct contact with a person or animal with that infection in the same way that other bacteria spread. Resistance is a particular problem in hospitals and places like old people's homes where many vulnerable people are gathered together.

Increased international travel means people infected with resistant bacteria in one country can spread them to another country very quickly.

Why don't we have enough new antibiotics?

Pharmaceutical companies are placing renewed emphasis on the search for new antibiotics and are also developing new vaccines to prevent common infections.

But these ventures are expensive to fund and in terms of cost-effectiveness for the company, may be less attractive than other business opportunities.

A course of antibiotic treatment is typically quite short - days or weeks - and doctors are advised to be frugal with their prescribing to help avoid resistance developing.

If major resistance does occur, the drug would be a write-off for the pharmaceutical company that developed it.

Many of the "newer" antibiotics are chemical variants of older agents and this means resistance development can occur quickly.

What can I do to help?

If you are prescribed antibiotics by your doctor, make sure you complete the course even if you feel better because not taking the full course encourages the emergence of resistance.

Remember that antibiotics are important medicines and should only be taken when prescribed by a health professional. They do not work for viral infections, only bacterial ones.

Don't share your antibiotics with anyone else.

Basic hygiene - hand washing and taking care when preparing food - can stop the spread of many bacteria, including some resistant strains.