VIRUSES
Viruses are the simplest living organisms. Even though they are the simplest, they were not the first forms of life, at least not the present day viruses. Viruses are obligate intracellular parasites and cannot replicate outside of a living cell. Every type of organism has viruses which parasitize them. Bacteria have viruses, we have viruses, fungi have viruses, plants have viruses....as far as we know all living things have viruses. Viruses are composed of two of the biomolecules we have learned about. They are composed of a nucleic acid, either DNA or RNA, and proteins. Very simple viruses like the tobacco mosaic virus which infects tobacco leaves, are composed of RNA and a single type of protein. The nucleic acid serves as their genetic material and contains enough information to enter the cell, replicate new viruses and escape the cell. The protein coat protects the nucleic acid and is instrumental in gaining access to the cell it infects. Viruses have no metabolism of their own and rely on the cell they infect to do all their work for them in constructing new viruses.
There are many kinds of viruses, some of them are fancy and some are very simple. The ones that infect bacteria are called bacteriophage (meaning bacteria eaters). They are often quite complex and may have many different proteins to help them get their genetic material (nucleic acid) past the thick bacterial cell wall and into the cell. Some of them have a protein in their stalk which can contract, just like the protein myosin in your muscles. The contraction of the stalk then acts like a syringe to inject the viral DNA into the host bacterial cell. Viruses and virus like particles called episomes are responsible for transferring antibiotic resistance from one bacterium to another. Sometimes viruses pick up a piece of the host DNA instead of their own and in that way carry genes from one bacterium to another (a process known as transfection). If the host bacterium is resistant to an antibiotic, the genes for resistance can be picked up and taken to another bacterium.
Antibiotics are middle-sized organic molecules which interfere with (prokaryotic) cellular metabolism. They are produced by other microorganisms to kill off their competitors. Fungi and bacteria are always competing for the same food sources. So they have come up with chemicals for biological warfare to get rid of competitors. Penicillin which was the first antibiotic used, came from a bread mold which is a fungus. Of course, modifications to these original antibiotics have been made in the laboratory so that we now have a wide variety. However, because of the misuse of antibiotics many of them are not longer effective because bacteria have acquired resistance to their action. The resistance is due to mutations in the disease causing bacteria which allow them to circumvent the action of the antibiotic. Antibiotics DO NOT work against VIRUSES. Antibiotics only work against bacteria. COLDS AND FLU ARE CAUSED BY VIRUSES, NOT BACTERIA. Therefore, antibiotics cannot cure colds or flu.
The only defense you have against viruses is your immune system. Viruses have proteins on their surface by which they enter your cell. (Some animal viruses take along a portion of the cell membrane when they leave your cells, too.) Some of the viral proteins attach to protein receptors in your cell membranes. Viral proteins are recognized as foreign by your immune system. The immune system contains white blood cells called T and B cells which make and secrete antibodies (proteins) to fight foreign molecules (including allergens), cells, and viruses. These foreign molecules, etc., are referred to as antigens. Antibodies are specific for each different antigen. And each antigen can stimulate the production of a number of different antibodies. The molecules on the surface of the invading cells or viruses are antigenic. The antibody molecules made by the T or B cells are composed of four polypeptide chains and the overall configuration resembles the shape of a Y. The tips of the two arms of the Y are the portions of the antibody that specifically attach to the antigen. Each T or B cell makes a different antibody. Each protein of the virus has many portions to its surface so many antibodies can be made to the same virus. When a virus invades your body or you are given a vaccine, your T and B cells recognize the virus or vaccine as foreign and begin making a variety of antibodies to capture (combine with) the proteins on the surface of the foreign cell or virus. The antibody molecules attach to the invaders and tag them for destruction by other members of your immune response team.
Vaccines are injections given to you to prevent a viral or other pathogen induced illness. The vaccine mimics the pathogen. Vaccines therefore may consist of just the viral protein coat (some flu vaccines), or they may be heat-killed viruses (the Salk polio vaccine) which can no longer replicate or a vaccine may be an "attenuated" live virus strain (Sabin polio vaccine). Attenuated viruses are those which have been grown in cell culture in the laboratory until they have mutated and can no longer cause the disease but they still have the same proteins as the virulent strain. Some of the antibody forming cells stimulated by the vaccine, are "memory" cells and when the "real" virus comes along, they are prepared to produce antibodies immediately to fight the infection. Vaccines can be developed to protect you from certain viral diseases but not all. Some viruses mutate very quickly and change their protein coats, making it difficult to develop vaccines for them. HIV is especially insidious because it specifically attacks the immune system.
Examples of human diseases caused by specific viruses include DNA viruses: papilloma causes warts; adenoviruses cause respiratory infections; herpes viruses cause fever blisters, chickenpox, shingles, mononucleosis, and genital blisters (after an infection, herpes viruses can hide in your cells for the remainder of your life); pox virus cause smallpox; hepatitis B; polio virus; and the rhino viruses are the most common cause of colds. We also have RNA viruses: influenza; measles; mumps; corona viruses cause colds; HIV which causes AIDS is a retrovirus; and hepatitis A.