INTRODUCTION TO VIRUSES

Description

It has been estimated that 60% of all human infections are caused by viruses. They are the simplest, yet least understood of all the microbes. However, new and better techniques to study viruses continue to evolve. Most viral infections are untreatable, and what "antiviral" drugs or vaccines that are available, are limited. The control of viral infections is difficult at best and more often than not, requires that prevention techniques be employed to prevent the transmission and spread of disease.

What are viruses? Are they alive?

Viruses are particles composed of genetic material, (e.g. DNA/RNA), lipids, (fats), and a protein wall which protects this material. Unlike bacteria, viruses lack the ability to grow or replicate on their own. Virologists have said that viruses are not "alive". They require a living host, (like a person), or a cell to support their replication. They are able to enter a cell and then take over that cell, directing it to make more virus particles. Some viral infections result in the eventual death of the host as in smallpox or rabies, while others allow the host to continue to live even after infection (as with the common cold).

Viruses are very, very small

They are about 1/100th the size of a bacteria or fungus. They are not even visible with a standard microscope. In order to be seen, a powerful electron microscope at magnifications of 10,000 to 100,000x must be used. Some virus particles are so small they cannot be seen. Their presence can only be detected by inoculating them into a susceptible host or tissue cell culture, and then looking for a characteristic effect, (e.g. illness in a person or death of the cell culture).

Classification of types of viruses

Viruses are classified 4 ways:

• based on their size and shape
• based on their genetic material (DNA or RNA)
• based upon how or where they replicate (in organs or cells)
• based upon their physical characteristics, i.e., lipophilic or hydrophilic.

(Lipophilic viruses are those which have a waxy or lipid shell, (or envelope), around them. These viruses are easier to inactivate with a disinfectant which can readily remove and destroy the lipid envelop. Lipophilic viruses include HIV, RSV and hepatitis B. Hydrophilic viruses do not have a shell but rather have a very tough protein coat that some disinfectants cannot enter. Hydrophilic viruses include poliovirus, rhinovirus and hepatitis A.)

How do we get viruses?

Viruses are spread or transmitted 4 ways:

• via the air in the form of very small droplets expelled from a cough or sneeze,
• by direct contact with infectious material such as respiratory or fecal material, or by person to person contact,
• by vectors, (bugs, mosquitoes, ticks, etc),
• by indirect contact such as touching surfaces that are contaminated.

Minimal infectuous dose

Viruses are shed in huge quantities when someone is ill, and it doesn't take very many of them to make you sick. For example:

• In one cc of nasal secretion there are estimated to be 1million (106) virus particles of RSV, and it takes 100-1000 of them to make you sick,
• In a gram of feces there are estimated to be 1011 virus particles of rotavirus, and it takes 1 to make you sick,
• In a gram of feces there are estimated to be 108 virus particles of hepatitis A, and it takes 1 to make you sick,
• In one cc of nasal secretions there are estimated to be 106 rhino-virus particles and it takes < 1 to make you sick. enough said!

In summary

1. Viruses are very different from bacteria and fungi and represent a unique infectious disease problem.
2. Most accounts of the incidence of viral infection are probably underestimated.
3. When "it" dries, "it" doesn't necessarily die.
4. In many cases only 1 virus particle may make you sick.
5. Viral transmission occurs many ways. Frequent hand washing and surface disinfection are important measures to help control the spread of disease.