I'll be talking about viruses in this blog and may be one or two others to answer some of Cherry's questions and discuss other points too. Please feel free to comment on any thing you read in the blog.

So what are viruses? The word "virus" itself is Latin and means poison.

Viruses are ultra-microscopic micro-organisms, obligate intracellular parasites that can infect wide variety of living organisms starting from humans till bacteria.

Size of virus:
Viruses are measured in nanometres (a metre is a billion nanometres) and they vary from 20-400 nms. Bacteria, on the other hand, along with higher organisms' cells are measured in micrometres which is a millionth of a metre.
In order to imagine the relative sizes of these cells you can say that a virus, a bacterium and a human cell put side by side are like a mouse, an elephant and a blue whale standing next to each others. (not very accurate but quite useful)

This is regarding the size which makes viruses the smallest creatures known, and this isn't the only special thing about them, there're some other aspects and we'll see that as we talk about the structure of viruses.

Viruses have very simple conformation. A typical virus or a "virion" will be containing:
A- Genetic material, not a nucleus or chromosomes but a small piece of nucleic acid. In all living cells other than viruses, the genetic material is made exclusively of DNA and they posses similar molecules named RNA which are, very simply, like an apparatus reading the code written in DNA language and transforming it to usable information ( sure we will discuss their mechanisms in other times).
In case of viruses, the genetic material could be made of DNA (like pox viruses) or RNA (like HIV)
B- A Coat made of protein which is a shell carrying the genome. It has another feature that its surface isn't smooth but rather carry some particles with specific shapes that help them recognise similar structures on their target cell (like a key and a key hole).
C- An Envelope made of fats and it isn't present in all viruses. Unlike what you might think the envelope doesn't give its viruses extra protection but it rather renders them more fragile as the fatty material of the envelope could be dissolved by any fat solvent like ether which leads to viral disruption.

From the structure of the viruses we can deduce their important classifications. They could be classified as: enveloped and non-enveloped viruses by the virtue that not all viruses posses enveloped. Also they could be classified into: RNA and DNA viruses according to the type of their genetic material.

There's another feature that is strange about viruses besides their very simple structure, very minute size and their peculiar genetic material. This feature is that a virus is only active inside living cells. That's to say, it doesn't perform any metabolism or multiply except inside cells and it's completely inert outside living cells. For instance, HIV (the virus causing AIDS) is transmitted from one to another carried inside white blood cells of the patient. This feature made some scientist call viruses semi-living aorganisms; that are only active iintracellular and are mere macro-protein particles outside living entities.

One last question I shall discuss..how are viruses harmful?
As we saw, viruses only exhibit life inside the cell of their victims. And what is their main goal? To make as many copies of itself as possible.. So a virus enters a cell be it a bacterium ,plant cell or whatever and uses all the resources, machinery and food of the cell to build hundreds or may be thousands of copies of itself which leads to depletion of the resources of a cell and its exhaustion. Now, the colony of viruses inside the cell can find no use of it so they destroy it to go outside and each one searches for another cell to build copies of itself and the cycle goes on and on... We can see one last peculiarity to viruses from their way of life. Their multiplication doesn't occur in the classical way as in animals or plants nor by simple fission as in bacteria and other unicellular creatures but rather by "replication".

That's all for now. In the next time we will see how the invaded organisms interact with viruses, how viruses can cause cancer and may be one thing or two about influenza viruses. Again, please feel free to comment or ask about any thing that has been mentioned in the blog.