Communicable diseases are spread by viruses and germs:

What is a germ?
The term 'germ' actually refers to any microorganism, especially those microorganisms that cause disease. Included in this category are certain viruses, bacteria, and fungi. What is the difference between these three types of microbes? Which ones cause which diseases, and should they be treated differently? Because viruses, bacteria, and fungi cause many well-known diseases, it is common to confuse them, but they are as different as a mouse and an elephant. A look at the size, structure, reproduction, hosts, and diseases caused by each will shed some light on the important differences between these germs.

What is a virus?
Viruses are very tiny, simple organisms. In fact, they are so tiny that they can only be seen with a special, very powerful microscope called an "electron microscope," and they are so simple that they are technically not even considered "alive." There are six characteristics of all living things:
  • Adaptation to the environment
  • Cellular makeup
  • Metabolic processes that obtain and use energy
  • Movement response to the environment
  • Growth and development
  • Reproduction

A virus is not able to metabolize, grow, or reproduce on its own, but must take over a host cell that provides these functions; therefore a virus is not considered "living." The structure of a virus is extremely simple and is not sufficient for an independent life.


Source (http://www.peteducation.com/article.cfm?c=18+1803&aid=2956)





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