Viruses

Viruses are even smaller than bacteria. They have some, but not all the characteristics of living organisms. They reproduce, for example, but they do not grow or eat. Viruses reproduce by entering cells, hijacking them, and using their cellular processes to multiply. As a result the infected cells often die. There arc thousands of varieties of viruses, and they cause many types of diseases. For example, the common cold, flu, warts, chicken pox, mumps, measles, and many gastrointestinal diseases are caused by viruses. Most viruses do not respond well to antiviral medicines -- and most don't need to be treated since they run their course and the patient recovers. A few anti-virus medicines are effective for serious viral infections.