Today I'll be completing what I've started in my last blog in the group about viruses, their general characteristics and how they're harmful. It would be preferable that you have read the first blog as there will be several points in my talk today that depend on what was mentioned.

Before getting to new stuff, I'd like to stress an important point regarding the structure of the coat covering the virus. I said that it could be a protein coat only or a protein coat and a fatty envelope. In either cases, this outer sheath be it the protein one or the fatty one, there are protrusions that appear on its surface. They're called surface receptors or surface 'antigens'. These are extremely important structures as they determine the type of cells the virus will attack. As I said they're like a key and the key hole is present on the surface of the attacked cells and the virus will only be able to invade a cell if it has the key that matches the cell key hole.
These particles are also important because the cells of the attacked organisms will recognise that it's a virus through them and can tell exactly which virus it is and thus can elicit a proper immune response. Cells of humans, animals, plants, fungi...etc have similar structures that serve similar function.

Now we shall continue the answer of the question: how are viruses harmful?

At the end of the previous blog we saw, generally, how viruses destroy their target cells. But now we shall see more specific examples of the viral damages:

• Viruses can inhibit the formation of protein and DNA in host cells which leads eventually to cell death

• Some viral proteins are inserted in the membrane of invaded cells which causes disruption of the membrane and consequently destruction of the whole cell

• These proteins that are inserted on host's cell membrane will make the cell seen by the immunity system as foreign and it will be attacked

• Viruses may cause damage to cells involved in antimicrobial defense thus paving the way to several infections. The classical example in HIV which depletes CD4+ cells which open the door for a hell of infections

• Virus killing of one cell type may lead to death of another type like in poliomyelitis where nerve cells are destroyed and thence muscle cells which aren't used at all die too

• A viruses can infect bacteria and doesn't form hundreds of copies of itself, but rather insert itself in the genome of the bacterium thus causing some characteristic changes in the bacterial cells. This causes harm in a remarkably interesting manner (of course interesting in an academic sense). An example will make this clear. Diphtheria bacteria are known to cause disease that could be fatal if untreated. The nasty effects of the infection are caused by a toxin the bacterium produces. It was found that only the bacteria that have viruses inserted in their genome secrete the toxin and the information for synthesizing that toxin is found on the genome of the inserted virus. Diphtheria bacteria that are virus-free were found completely harmless

• CANCER

Viruses are, almost, the only organism that has been found to have definitive role in the causation of cancer. The way through which they do so is mostly understood.
The mechanism has some resemblance to what we have seen in the interaction between the diphtheria bacteria and viruses. Some types of human viruses, instead of taking the usual path and forming multiple of copies of themselves, they tend to insert themselves in the genome of the cell and settle their. In the bacteria, it wasn't a big deal and it didn't interfere with the host well-being. In case of human cells, the situation is completely different and this is logical because the human structure is radically different. Cells are governed by strict rules posed by the nervous system, hormones and many other local factors.
Upon the insertion of the virus in the human cell genome, all these rules will be disobeyed. The cause is that the genome is a perfect code that guides a cell to its role and addition of foreign letters inside the code will disrupt it. So the affected cells obey no rules and divide endlessly leading to the infamous disease. It would be improper to say that any virus that takes this path will cause cancer. In some cases, the insertion of the viral genome amongst the human code causes intolerable changes that are incompetent with cell life. For a virus to cause cancer, it has to be implanted in very special sites that code for enzymes that regulate cell division and prevent cancer development. In such a case, the enzymes will be defective or not formed at all opening the gates for cancer.
Few types of viruses have the ability to put themselves in these sites and they're known causes for cancer and here are some examples:

1. Human T-cell leukaemia virus type 1 (HTLV-1) which is associated with leukaemia/lymphoma

2. Human papilloma virus that has been implicated in the causation of squamous cell carcinoma of the cervix of uterus

3. Epstein-Barr Virus which has been described to be causative for some types of lymphomas…and there're other examples

I think this is enough for now. In the next post we will see a very interesting issue; that is how the immunity reacts to viral infection. I hope to see some comments coz this means that someone has read the post!