It's human nature to seek explanations for the things that happen to us. So when we get an infection, we want to know where it came from and why we got it.

It's often impossible to be sure. Infections are caused by microscopic organisms - bacteria, viruses and fungi - so we can't see them to track how they spread.

Even if we could, it wouldn't be possible to recognize all the dangerous ones among the billions of harmless micro-organisms that surround us.

Scientific detective work over the years has revealed where certain harmful microbes lurk and how they're passed on. This helps doctors to make informed suggestions about how an infection's been picked up, but it's just guesswork.

Only careful laboratory tests can trace the source of an infection with any certainty.

Some health care settings, especially hospitals, are more likely to harbour dangerous micro-organisms. Patients are particularly vulnerable to infection because their immune systems tend to have been weakened by disease.

Worse still, the organisms that tend to survive in hospitals are those which have developed resistance to antibiotics.

Health care staff and visitors carry the bacteria around - 30 per cent of people are carriers of dangerous infectious organisms.

Prevention

In future, with better scientific understanding and more sophisticated laboratory tests, we may know the answers to where harmful infections come from. Until then, it's a case of taking all the necessary precautions to reduce your risk of infection.

• Wash your hands regularly (this is also vital to reduce the chance of passing on an infection to others)
• Keep your immune system strong by following a healthy diet, taking regular exercise and getting plenty of rest
• Store food carefully and cook it properly
• Practice safe sex
• Keep away from identifiable high-risk situations, so avoid friends with flu and be extra vigilant while in hospitals