American Medical Association Family Medical Guide - American Medical Association [643]
Viruses and Bacteria
Viruses and bacteria are the two microorganisms that cause most infections. Viruses cause infection by taking over the machinery of the cells they invade and making copies of themselves. In the process, the host cells are usually destroyed. The newly manufactured viruses invade other cells, continuing the cycle of multiplying inside the cells and destroying them. Viral infections may cause symptoms such as high fever, muscle pain, headache, and weakness. There are no effective treatments for most viruses, but some viral infections can be treated with antiviral drugs.
Bacteria live everywhere, including in and on the human body. Many types of bacteria are helpful in protecting the body from disease. They inhabit the skin and the mucous membranes that line the mouth, nose, intestines, and vagina, acting as barriers against potentially harmful invading microorganisms. Problems arise, however, when the beneficial bacteria in your body multiply in greater-than-normal numbers and cause infections or when harmful bacteria enter your body, such as through a cut in the skin or in food you eat. Some bacteria cause disease by producing toxins that damage cells.
Most of the harmful bacteria can be eliminated with antibiotics. However, antibiotics must be used appropriately, only for bacterial infections, and exactly as the doctor prescribes. If you stop taking a course of antibiotics before you finish the prescription, you risk having a recurrence of the infection. If you take antibiotics frequently or intermittently, you risk allowing bacteria to grow that will be resistant to the effects of the antibiotic and therefore harder to eliminate. Antibiotic resistance can also result when antibiotics are taken for viral infections such as a cold. Antibiotics are not effective against viral infections.
How viruses multiply
Viruses cause infection by invading cells in the body and taking over the cells’ internal machinery to make copies of themselves. The viruses usually destroy the host cells in the process. The newly formed viruses leave the host cells, and go on to invade other cells, continuing to multiply.
How bacteria multiply
Bacteria are more self-sufficient than viruses. They reproduce by dividing in two. Each of the new cells then divides in two, and the process continues. Bacterial reproduction is so efficient that 1 million bacteria can develop from a single bacterium within 10 hours.
Some infectious organisms are spread when they are coughed or sneezed into the air and breathed in; others are spread through physical contact such as a handshake. Still other organisms are spread in contaminated food or water. Some microbes are transmitted to humans from animals or animal products (such as meat or eggs). If the spread of an organism that causes an infection can be stopped, the incidence of the disease may be reduced.
The parasites that cause infestations—such as worms, ticks, and lice—are larger than bacteria and viruses, and can be seen without a microscope. Infestations usually cause symptoms only at the site of infestation.
Generalized Infections
The infections in this section cannot be categorized easily, but one thing they have in common is that they produce symptoms throughout the body. Infectious mononucleosis and shingles are caused by viruses, while tetanus and toxic shock syndrome are caused by bacteria.
Infectious Mononucleosis
Infectious mononucleosis, often called mono for short, can be caused by either the Epstein-Barr virus or cytomegalovirus (see page 508). The infection can be transmitted from one person to another through oral contact such as kissing, and can spread to almost any organ. Mononucleosis is more common in teenagers and young adults (especially those who