Most of us can't tell the difference between viruses and bacteria. After all, they're only germs. Even the most experienced scientists in the early 20th century didn't know what viruses were. You might be surprised to learn that viruses weren't fully understood until the 1950's, when the electron microscope was developed. It is now known that viruses can cause diseases like colds, influenza, measles, hepatitis, meningitis, yellow fever, herpes, smallpox, AIDS, Epstein Barr syndrome (i.e. Mono), and even tumors.So, how are they different? Let's take a look at their viral characteristics.
A big difference between viruses and bacteria is that most viruses are a lot smaller than bacteria. As you see, The foot-and-mouth Virus is only 24 nanometers long! This means that you can put over 400,000 of them into a line and their total length wouldn't even reach a millimeter. Both bacteria and our body cells are much bigger than viruses.
Viruses are generally one of three shapes. They can be helical, like the Tobacco Mosaic Virus (labeled TMV). These viruses have a helix of RNA, or its blueprint, with a protein coat around. They can also be polyhedral, like the Adenovirus. These viruses are shaped like a many-faced dice. Lastly, viruses can have an enveloped structure, in which the virus' blueprint is inside a spherical membrane.
|The little "spikes" on the surface of the viruses shown are called antigens. This allows viruses to recognize, attack, and penetrate a specific cell. Viruses can be thought of as blind men, with keys in their hands, roaming around in a huge hallway of locked doors. The antigens the virus has are also what enable our immune system to search and kill them.|
It's not what viruses have, but what they don't have that describes and characterizes them. Viruses are missing all of the chemical structures and molecules that allow cells to survive and spread. Without them, anything would die. Viruses can't self-replicate, or reproduce alone, and they can't grow. The only way viruses can replicate is if they take over another cell and force it to make copies of that virus. That's why they must rely on us in order to survive and spread. If you think about it, our infection is their sex!
- How Viruses Spread
Now, once the virus as made thousands of copies of itself in the cell, it now has to infect other cells. There are three possible ways in which the newly formed viruses exit the cell:
1. The viruses can force the cell to create so many copies of itself that the cell eventually explodes, or lyses. The viruses are then released, and the cell is dead.
2. Another way is do become dormant, or lysogenic. These viruses just "sleep" in the cell until some type of stimulus excites them, forcing them to lyse the cell.
3. Some viruses are persistent viruses, meaning that they exit the cell without killing it. This is like HIV.
Let's Review1. Viruses are almost always smaller than bacteria, and not visible in light microscopes.
2. Viruses contain either DNA or RNA, and a capsid (or a protein shield) to protect itself. DNA and RNA are like blueprints it uses to replicate.
3. Viruses can't replicate themselves- they need us! They can't grow, and they can't divide until they are in a cell.Well, because viruses are so deadly, we have an immune system to help protect ourselves from sickness. Think of our immune system as our army. Our immune system has primary, secondary, and tertiary defenses. Let's take a look:Our primary defense is our skin. Viruses generally must enter though natural body openings or through cuts or scatches. Such body openings are the mouth, eyes, ears, and nose. But our immune system has this covered through our secondary defense. We have mucous to help trap and kill germs which come through our respiratory tract. We have saliva and acids in our stomach to protect ourselves from germs which go down our digestrive tract. Finally, we have tears and earwax to trap and wash away anything that goes into our eyes or ears. If the virus makes it past our 1st and 2nd line of defense, our 3rd line of defense is put to action. Our tertiary line is divided up into two groups, innate immunity and acquired immunity. Innate immunity, or defenses that don't care what type of foreign element they kill, consist of phagocytic cells, natural killer cells, and interferons. Phagocytic cells are white blood cells that look for any type of bacteria, virus, or dead cell and eats them. Think of them as garbage collectors. Phagocytes also clean up wound sites. Natural Killer cells, or NK cells, differ from phagocytic cells in that instead of eating the foreign element, they attack and destroy our own cells that have already been infected by the invading germ and are abnormal (cancerous). Again, these respond to any type of infection. Lastly, we have something called interferons, which are a little harder to understand. If a cell is attacked by the virus, the cell releases interferons which bind to other healthy cells, creating an "antiviral state," or a "second shield." It's like a cell sacrificing all of its defenses to it's ally when it dies.If this doesn't stop the virus, then our last line of defense is activated. Instead of being "blind," this part of the immune system, called acquired immunity, looks for antigens, or the receptors on the surface of the virus, and attaches on to them. Attaching to the virus does 3 things. First, it neutralizes the virus, or covers the surface of the virus in such a way that the virus can't attack another one of our cells. It also assists in the process of phagocytosis- in other words, it helps devour the virus. Finally, it destroys the virus itself by eating away the viruses "skin."Once we've defeated the virus, one of our immune system's greatest features is that we will be able to remember the virus forever. This protects us if the same virus wants to enter again in the future. If this happens, our immune system will immediately attack the virus, killing it at a much quicker rate. This is why vaccines are so effective. Vaccines trick the immune system into thinking that it has already seen the virus.
So here we are with an extremely sophisticated and effective set of defense mechanisms protecting us from continued viral infections. Then why is an HIV infection considered to be permanent?Shouldn't those cells infected with HIV be susceptible to our killer cells? And shouldn't the immune system be able to produce antibodies to the HIV antigens? The good news is yes, the T8 (killer) cells and the specific antibodies do carry out their special responsibilities.The T8 cells do destroy the cells that are infected with HIV, and the antibodies can destroy free HIV virus in the blood stream. The very very bad news is that the specific cells that HIV attacksare the T4 cells and macrophages!! Both are major parts of our immune system. Talk about the best defensebeing a good offense! HIV has taken the most obvious evolutionary step of all, attacking the very cellsresponsible for our defense against viral infections! In this case, when the T4 cells and macrophagesare instructing the T8 cells to destroy any cell infected with HIV, it is only a form of cellular patricide,since it's its very own brother T-cells that are infected!
-Picture 1 of AIDS
-Picture 2 of AIDS
-Picture 3 of AIDS
1. The Infection is Forever
2. The Infection Can Be Latent (it can 'sleep');
3. The HIV attacks Human Immune System!