As you might think, bacterial infections are caused by bacteria, and viral infections are caused by viruses. Perhaps the most important distinction between bacteria and viruses is that antibiotic drugs usually kill bacteria, but they aren't effective against viruses.

Bacteria

Bacteria are single-celled microorganisms that thrive in many different types of environments. Some varieties live in extremes of cold or heat. Others make their home in people's intestines, where they help digest food. Most bacteria cause no harm to people, but there are exceptions.

Infections caused by bacteria include:

• Strep throat
• Tuberculosis
• Urinary tract infections

Inappropriate use of antibiotics has helped create bacterial diseases that are resistant to treatment with different types of antibiotic medications.

Viruses

Viruses are even smaller than bacteria and require living hosts — such as people, plants or animals — to multiply. Otherwise, they can't survive. When a virus enters your body, it invades some of your cells and takes over the cell machinery, redirecting it to produce the virus.

Diseases caused by viruses include:

• Chickenpox
• AIDS
• Common colds

In some cases, it may be difficult to determine whether a bacterium or a virus is causing your symptoms. Many ailments — such as pneumonia, meningitis and diarrhea — can be caused by either bacteria or viruses.