Q. I had a bad cold so I asked my doctor for an antibiotic. He seemed reluctant, but I insisted and he gave me the prescription. I was supposed to take it for 10 days, but I stopped after seven days because I felt better and I ...

Stop! Next you'll tell me you prefer not to cover your mouth when you cough.

Taking antibiotics unnecessarily and not completing your prescription are the leading causes of "superbugs," bacteria that are resistant to antibiotics. These superbugs are one of the most serious threats to global public health.

The first thing you should know is that antibiotics are used to combat bacteria, not viruses. So, these potent drugs should be used for infections of the ear, sinuses, urinary tract and skin. They're also used to treat strep throat. They should not be used for viruses that cause most sore throats, coughs, colds and flu.

Doctors in the United States annually write about 50 million antibiotic prescriptions for viral illnesses anyway. Patient pressure is a major cause for these prescriptions.

When you don't finish your prescription, your antibiotic doesn't kill all the targeted bacteria. The germs that survive build up resistance to the drug you're taking. Doctors are then forced to prescribe a stronger antibiotic. The bacteria learn to fight the stronger medication. Superbugs are smart, too. They can share information with other bacteria.

The antibiotic vancomycin was, for years, a reliable last defense against some severe infections. But some superbugs have even figured out how to resist vancomycin.

More than 70 percent of the bacteria that cause hospital-acquired infections are resistant to at least one of the antibiotics most commonly used to treat them. About 100,000 people die each year from infections they contract in the hospital, often because the bacteria that cause hospital-acquired infections are resistant to antibiotics.

Here's what you can do about this problem:

Protect yourself by washing your hands often, handling and preparing food safely, and keeping up-to-date on immunizations.

Take antibiotics exactly as prescribed. If you cut your treatment short, you kill the vulnerable bacteria, but allow the resistant bacteria to live.

Never take leftover antibiotics from your medicine cabinet or from a friend. The antibiotic might not be right one to use. And, if it is, you probably won't have enough pills to kill the germs in your system. This can lead to more resistant bacteria.

Don't pressure your doctor for antibiotics if you have a viral illness.

Penicillin was the first antibiotic. It was derived from mold. We now have more than 150 of these drugs. Antibiotics are a class of antimicrobials, a group that includes anti-viral, anti-fungal, and anti-parasitic drugs.

Previous treatments for infections included poisons such as strychnine and arsenic. When antibiotics arrived, they were called "magic bullets," because they targeted disease without harming the host.

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