Let's start today by talking about some of the ways through which viruses are transmitted from a host to another. We should understand that the aim of a virus, like all parasitic and non-parasitic organisms, the aim is to reproduce; that's to say to make multiple 'copies' of itself. So, it should find the suitable environment for growth and replication (inside living cells) and also an appropriate mode to transport itself to another host. It isn't logical at all for a virus to kill its host as this will cripple its life cycle. SO a virus isn't vicious itself but the damages it causes come either as a byproduct of its life style or as reaction from the invaded host.
This long introduction is essential to understand how important the modes of transmission are to viruses. The numerous ways of transmission are significant of the ingenuity of these ultra short organisms in accommodating to various circumstances.
So among the important modes are:
• Respiratory route: through droplets that are coughed or sneezed from a patient or a carrier like influenza viruses
• Blood transfer: virtually all viruses that appear in blood can be transmitted this way either in transfusion or using contaminated syringe needles like Hepatitis C virus (which, most unfortunately, is highly endemic in my country Egypt)
• Faeco-oral route: and this is associated with poor hygiene were insects tend to transmit the virus from faeces of patients to food and also in overcrowded places where there's no proper distinction between bathrooms and places of eating! and example of this is polio virus which causes paralysis in infants
• Mosquitoes: which transmit some tropical diseases like yellow fever which is caused by arbo virus
• Exchange of saliva like in kissing and this occurs in Epstein-Barr virus
• Sexual intercourse: like in HIV
• Direct contact with oozing lesions like warts as in Herpes viruses
• Bite of a rabid animal which transmits rabies

Now we shall discuss a very interesting topic which is the response of the immunity to the infection. In fact immunity is one of the most amazing mechanisms that occur in the body and the extraordinary coordination between its elements is one of the marvels of the universe. It's a very wide topic but we will have an overlook on it.

The defences of the body start before an organism enters the body. The skin, for instance, is resistant to invasion by any microorganism as it's covered by vert tough layer called keratin. Also, the stomach secretes very strong acid with pH about 2. This acid kills most of organisms that are eaten with food and inhibits any organisms from living it and it was for long thought that there're absolutely no organisms living in the stomach until an Australian scientist discovered 2 decades ago that H. pylori, a bacterium, could live in the stomach and cause gastritis and ulcer. The respiratory tract also has mucous on its surfaces where inhaled particles stick to it and are swept away by cilia pushing to the outside. Tears, saliva, sweat and other secretions contain hydrolytic enzymes which attack invading organisms.

When these defences fail and a virus enters, various cells roll their sleeves and start action. We have to know that all their functions are done according to surface receptors that various organism posses. I have mentioned some stuff about these structures in the last post so refer to them if you haven't read them before.

Anyway, there are the "phagocytes" or the eater cells and the master of these are the "macrophages" or the big eaters. These cells eat and digest many foreign organisms and then digest them. Not only do they attack foreign cells, but they also eat aged native calls that can't function properly anymore.

Then comes the "natural killer" cells or NK cells. These cells secrete toxic substance upon encountering a cell that has strange combination of receptors on its surface. It was found that a virus that infects a cell leaves its receptors on the surface of a host cell. So an NK cell attacks any virus infected cell. This explains to us the dilemma that doctors find when they want to transplant an organ to a patient from some one else, because they should find some one whose cells bear extremely similar receptors as those of the patients' or else the NK cells and the other cells will attack it.
Further more, the infected cell secrete INTERFERONS. These are thought to be the primary defence in case of viral infection. Their name, interferons, implies that they interfere with something. What happens is that when a virus, any virus, attacks a cell, this cell secretes interferons go to neighbour cell and adhere to their surfaces. This tells them to synthesize certain substances that will interfere with replication of viruses during their forthcoming attack. This of course limits viral dissemination. Because of their potent effect, they have been a major drug in tackling viral diseases and now they're one of the most important products of genetic engineering industry together, of course, with insulin.

These cellular reactions together with the defences that prevent the organism from invasion have been called (Natural or Non-Specific Immunity) and this is because they don't require any previous knowledge with the organism and are present since birth. And this will be clear when we see the (Specific or Acquired Immunity) which is the second arm of immunity.

The specific immunity requires previous knowledge with organism to elicit a specific response; this response is either humoral and it's mediated by B lymphocytes or cellular and performed by T lymphocytes. In both cases the cascade of events is started with our friends, macrophages that we have met 4 paragraphs ago.

Besides what macrophages do as 'eater' of the viruses, they put the receptors of these viruses on their surface and present them to T lymphocytes (T cells for short). It's supposed that 1 of every 10,000 T cells in the blood react to these macrophages. Anyway, these T cells that recognize the receptors will secrete stimulating substances that will activate other T cells that can deal with this virus and also tell them to multiply so that we have a colony that will attack the virus-infected cells to destroy with their viruses inside. Also these activated T cells will secrete interferons, and we know now what interferons do. A third set of secretions are directed to B lymphocytes (B cells for short) that are able to react to the virus to activate them.

The activated B cells are responsible for secreting antibodies that are directed to the free viruses (virions) and to the viral infected cells. These antibodies have many actions. And by the way they have some role to do with allergy but that's a separate subject. Anyway, these antibodies (also called immunoglobulins) first cover surface receptors of free viruses thus rendering them unable to attach to any cell and thus infect it. This called neutralisation of the virus. This could be understood when you know that these antibodies are Y-shaped and they cover the receptors by the "v" portion.
The antibodies also surround the cells infected with viruses in the same manner with free viruses with the "v" portion of the antibody. This will make these cells more vulnerable to NK cells and macrophages and will facilitate targeting them.

That's too much for one post so I'll continue in the next post with a relevant subject that is dealing with infection from outside, i.e. vaccination and chemotherapy.

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!