Home            Viruses

Viruses are sometimes regarded as the simplest known living things although other people do not accept that they are living.  This comes down to defining what living means and there is no universally accepted definition.

The viruses that have been discovered so far have all been parasites that enter a cell and change the cell’s instructions so it produces copies of the original virus.

Free Living Viruses

No free living viruses are known.  However, if they existed, finding them would be an extremely difficult job.  With disease causing viruses, their presence is suspected from their effects on the host they are infecting.  Even then finding the virus is a difficult job.

However, if, for example, there were some free living viruses in the ocean, the job of finding them would make finding a needle in a haystack seem very easy.

But although no free living viruses have been found, it is entirely possible that they do exist. Even if they do not exist, they may have existed in the past.

For a virus to be free living it would have to replicate its nucleic acid from nucleotides, ribose sugars and phosphate groups.  Nowadays there is possibly no environment with consistently enough of these.  Having replicated its nucleic acid the virus would then need to assemble its protein sheath.  However, the main use of the protein sheath is to enter cells.  So perhaps a free living virus might consist of only self replicating nucleic acid.  This could be much simpler even than the viruses that are known.

Of course someone might argue that this hypothetical entity was not a virus. Again this is a matter of definition.

Where do viruses come from?

We do not know how they originated and there are several different theories.  One of these theories is implied by the previous paragraph. In the early seas and other water bodies of the Earth, there may have been quite a lot of nucleic acids and other organic compounds including ribose sugars and phosphate groups.  Some chemicals could form capable of using the simpler organic compounds to make copies of themselves.  This is what DNA does.  These self replicating chemicals would not be what most people would call life. They would not always make perfect copies of themselves.  Competition would develop between these self replicating chemicals for the organic building blocks.  The self replicating chemicals most efficient at using them would make more copies of themselves.  So these pieces of DNA would develop into what we might call a free living virus.

This might have happened before there was any cellular life.  Perhaps when cells started to appear some of the free living viruses started to enter the cells and were in an ideal environment to self replicate. Maybe it was at this point that the protein sheath became an advantage to enable the nucleic acid of the virus to be injected into the cell.


In recent times these parasitic type viruses have become an important disease causing agent. Effective antivirus drugs are a quite recent development.  The most important specific preventative for viruses is still vaccination.

General Health

Many common viruses such as the common cold cannot yet be vaccinated against.  A well balanced diet containing enough fresh fruits and vegetables and other foods will ensure that our bodies have the nutrients needed for our immune systems. Another factor is to avoid excessive stress.  Someone who is stressed is more likely to get ill. 


History of virus:










Transmission electron micrograph image showing an Orf virus, a member of the genus Parapoxvirus.

The Orf virus is a pathogen common to sheep and goats, but can at times be transmitted to humans.When a human being becomes infected, the symptoms are mainly confined to the skin, and consist of “weeping” nodules on back of the hands, and which heal in approximately thirty-five days.

Photo credit: Cynthia Goldsmith
Source:CDC's Public Health Image Library Image #8434
President Franklin Roosevelt was infected by polio viruses when he was young.  Nowadays Polio can be prevented by vaccination.