Part of what are commonly referred to as "germs," infectious bacteria invade the human body, triggering an immune system reaction. When the body is unable to wipe out the pathogen, antibiotics are often administered to return to health.

Bacteria are one-celled organisms that sometimes cause disease. Not all bacteria are harmful to humans; there are many types that live in and on our bodies without causing infection; such as the bacteria on our skin that helps break down dying skin cells or the bacteria in our intestines that break down waste into simple substances.

Antibiotics are administered to humans to combat the presence of bacteria that are causing infections. They are taken orally or injected into muscle. Antibiotics are ideally administered at a high enough concentration to fight the infection during an extended duration and totally wipe it out.

There are many different types of antibiotics: some are specifically suited for destroying certain bacteria, others kill many types of bacteria. In order to kill bacteria, the antibiotic may spoil the structure of the bacterium cell wall, prevent energy receiving processes or disrupt production of essential proteins.

Occasionally, a few bacteria are able to survive the antibiotic's attacks and live to reproduce. Typically, a newer and stronger generation of bacteria can be produced in as little as 15 minutes. The new strain of the resistant bacteria can multiply quickly. The more humans use antibiotics, the more often resistant strains of bacteria are produced. Today, almost every known disease-causing bacterium has developed an antibiotic-resistant strain.

The Food and Drug Administration says antibiotics can be used in pets and farm animals. There are more than 115 million dogs and cats, 6.9 million horses, 7.8 billion chickens, 292 million turkeys, 106 million cattle, 101 million pigs and 8 million sheep in the United Sates. Livestock producers use antibiotics to protect farm animals from disease and as growth promoters.

The FDA first approved the use of antibiotics - some of the same used to treat humans - in farm animal feed in 1951. According to a survey by the Animal Health Institute, 20.5 million pounds of antibiotics were sold for animal use in 1999. Of that, 17.7 million pounds were used for treating or controlling disease and the remaining 2.8 million pounds were used for growth promotion.

There is growing concern that bacteria living in farm animals can develop resistance to the antibiotics. Farm animals eat feed laced with antibiotics, and in some cases they are given a sub-therapeutic dose of the antibiotic: not enough of the drug to kill the bacteria, but enough to make it resistant. The bacteria are able to mutate into a stronger strain and survive in the animal.

The animals are slaughtered with the now antibiotic-resistant bacteria still in the meat. Humans then consume the meat that contains strains of the drug resistant bacteria.

Eating the meat then transfers the food-borne and other bacteria to humans that are now resistant to the same drugs that a doctor would prescribe to treat the illness. According to the Union of Concerned Scientists, of the 56 antimicrobial drugs being given to animals, 34 of them are also used to treat humans.

Alexander Fleming, a Scottish scientist, discovers the first antibiotic, penicillin, but is unable to isolate it in its purest form.

Antibiotics come into worldwide prominence when concentrated penicillin is produced by biochemist Ernst Chain.

Enough penicillin is available to treat wounded soldiers and severely ill civilians in North Africa and Europe during World War II.

The Food and Drug Administration approves the use of antibiotics in farm feed, many antibiotics approved are the same ones used to treat humans.

Scientists add various chemical groupings to the core of the penicillin molecule to generate semi-synthetic versions.

The first penicillin-resistant strain of pneumonia is reported in New Guinea.

News reports reveal that many diseases have become resistant to antibiotics. 25% of bacterial diseases can only be treated by the strongest antibiotics.

The FDA proposes a framework for evaluating the safety of antibiotics for livestock and its capacity for promoting antibiotic-resistant bacteria in humans.

Four antibiotics are banned for use in animal feed by European Union (EU) agriculture ministers because of concerns that their entry into the food chain could increase the risk of bacterial resistance to certain drugs in humans.

A government advisory committee recommends that the FDA approve a new type of anti-biotic to fight drug-resistant bacteria. It's called Zyvox, and it's designed to combat pneumonia, skin problems and other hard-to-treat bacterial infections.

Human Health Services unveils a plan to combat antimicrobial resistance.

The Centers for Disease Control and Prevention (CDC) says there are two main types of germs that cause most infections: viruses and bacteria. Antibiotics are powerful drugs used to treat certain illnesses, but they don't work against viruses such as colds and flu, but they do kill specific bacteria. The CDC cautions that antibiotics do not cure everything, and unnecessary use of antibiotics can even be detrimental. But if your illness is treatable by antibiotics, remember that there are pros and cons of using these drugs.

- Strep throat

- Urinary tract infections

- Most ear infections

- Some sinus infections

- Cold

- Flu

- Sore throat

- Minor cough

- Don't stop taking antibiotics if you are feeling better. If you stop taking the antibiotic early, the surviving bacteria may restart an infection.

- Don't save antibiotics for later use or share them. Using a part of a prescription could allow bacteria to survive and re-infect you.

- Don't go to a doctor expecting or demanding antibiotics. Many times, it's best to take medicine to alleviate symptoms and allow the body's immune system to fight an infection.

Source: CDC. If you have any questions, contact your doctor.

In a three-year study by the Centers for Disease Control and Prevention, thousands of samples of Streptococcus pneumonia were tested against various classes of antibiotics. Strains that were resistant to penicillin increasingly became resistant to other widely used antibiotics.

According to the Animal Health Institute, in 1999, 17.7 million pounds were used for treatment and prevention of disease and 2.8 million pounds were used for improving feed efficiency and enhancing growth.

According to the Union of Concerned Scientists, total nontherapeutic antibiotic use in animals has increased from 16.1 million pounds in the mid-1980s to 24.6 million pounds today. 

Nontherapeutic use of antibiotics kills some bacteria, but other bacteria survive to become stonger and more resisant to antibiotics.