Viruses

Viruses are not alive. Actually, viruses have some characteristics of living things, but not all of them. Viruses are also incapable of carrying on these proceses without the help of living cells. They're dependant upon cells to reproduce. Because they do reproduce, they have nucleic acids and effect living organisms so much, they are studied with the world of life, but again, they're not really alive. Well, most of the time.

The word virus is the Latin word for poison. Many viruses are disease causers and all of them can be a problem in the right conditions. Many of the nasty diseases on Earth are caused by viruses. Even some plant diseases are caused by viruses. Not even bacteria are immune. Specialized viruses, called bacteriophages can invade a bacterium.

Viruses are little more than some DNA or RNA wadded up in a little container. Once inside a cell, the DNA or RNA uncoils and worms its way into the genetic code of the cell. This added nucleic acid gives instruction to the cell to make more viruses. Here's a comparison for you. Imagine a child going with a parent to the grocery store. Let's say the child writes a few items down on the shopping list, and the parent doesn't notice and buys them anyway. A similar thing is happening. The cell follows the directions handed to it and in turn makes more viruses. In this way, the virus can use the cell to make more little viruses.

If the virus and the host have a long history together, like humans and the common cold, the virus won't kill the host, but make more viruses for a while and the host's immune system will eventually get rid of it. Sometimes, the virus is so quick acting, or so nasty, or crippling to the immune system that the host cannot kill off the virus before the virus kills the host. This results in fatal diseases like ebola, HIV induced AIDS and many other nasty diseases.

But when a virus isn't in a host, it is said to be dormant. It cannot reproduce at all. This is why viruses aren't really counted as living organisms. In spite of this, biologists insist on classifying them. They group them according to their shape, their size and their nucleic acid. A table showing the types of viruses follows.

Characteristics	Viral Family	Shape	Diseases
DNA Viruses
Single strand,no envelope	Parvovirus		Stomach problems, fetal death
Double strand, no envelope	Adenovirus		Respiratory infections, animal tumors
"      "	Papillomavirus		Warts, some cancers
Double strand, envelope	Orthopoxvirus		Smallpox, cowpox, etc.
"      "	Herpesvirus		Herpes, Chickenpox, Smallpox, chickenpox, fever blisters, mononucleosis, etc.
"      "	Hepadnavirus		Hepatitis B, liver tumors
Complex	Bacteriophages		These invade bacteria cells
RNA Viruses
Single strand, no envelope + strand	Picornavirus		Polio, hundreds of cold (rhino) viruses
Single strand, envelope + strand	Togavirus		Rubella, several encephalitises
"      "	Flavivirus		Yellow fever, several encephalitises, Hepatitis C
"      "	Coronavirus		Respiratory tract infections, some colds
- strand, single strand	Rhabdovirus		Rabies and numerous animal diseases
" "	Filovirus		Ebola and Marburg
" "	Paramyxovirus		Mumps, parainfluenza, other diseases
- strand, multiple strands	Orthomyxovirus		Red blood cell problems
" "	Bunyavirus		Hantaviruses and some influenzas
" "	Arenavirus		hemmorhagic fevers, lymphocytic choriomeningitis
DNA producer	Retrovirus		HIV, leukemia, some tumors
Double-stranded RNA, no envelope	Reovirus		Colorado tick fever, mild respiratory infections, others

Virus shapes and format adapted from Microbiology, An Introduction, 5th Edition, pages 339-340. Copywrite 1995 Benjamin/Cummings Publishing. However, I made the graphics and everything myself. I just thought it would be nice to mention the book as a mentoring source.

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