Back To CourseCLEP Biology: Study Guide & Test Prep
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In 1900, the average life expectancy in the United States was 48-51 years, and the top three leading causes of death were pneumonia, tuberculosis and diarrhea, all of which were caused by infectious diseases. However, by 1997, the average life expectancy rose to 74-80 years, and pneumonia and influenza combined were the sixth leading cause of death and the only infectious diseases to crack the top ten. Many once fearsome and devastating diseases, like smallpox, polio, syphilis, tuberculosis, measles, cholera and the bubonic plague, have either been nearly eradicated or are now easily treated with modern medicine. The dramatic increase in the average life expectancy, not only in the U.S. but around the world, in the last 160 years has been accomplished in large part due to the effective prevention and treatment of infectious diseases through the use of sanitation, antibiotics and vaccines.
Experiments performed by Louis Pasteur in the 1860s convinced the Western scientific community that the germ theory of disease was correct. The germ theory of disease is a scientific theory which states that infectious diseases are caused by microscopic organisms that must be transferred from one host to another in order to spread. The acceptance of germ theory led to wide-scale changes in medicine, food preparation and waste disposal. Prior to the 1860s, medical practices were extremely unsanitary. Washing hands and instruments before and after surgeries was not a common practice, and the more dried blood and gore that a surgeon had on his surgical clothes, the more prestige and experience he was assumed to have. However, after the realization that disease was caused by microorganisms that could be transferred by body fluids, food and waste, wide-scale changes in medicine, food preparation and waste disposal were implemented in the Western World. Death rates from infection in hospitals fell, and between 1850 and 1900, the average life expectancy in the U.S. jumped up by more than 25% from 39 years to almost 50 years.
Another breakthrough in medicine that has contributed to the rise in life expectancy over the last 160 years is the widespread use of vaccines. Vaccines are non-disease causing variants, or derivatives of pathogens, that are used to create immunological memory against the pathogens themselves. Strangely enough, the first successful vaccination to be tested against a challenge with a pathogen was performed in 1796 by Edward Jenner, more than 60 years before the germ theory of disease was widely accepted by the scientific community.
It had been observed that milk maids did not often contract smallpox, which was in sharp contrast to an estimated 60% of the general population that contracted the disease. However, almost all milk maids did contract a related, but nonlethal disease, called cowpox, from the udders of the cows that they milked. It was postulated that contracting cowpox made a person resistant to smallpox infection, so Jenner took it upon himself to test this hypothesis. He took pus from the cowpox blisters of a milkmaid and inoculated a young boy with it. The inoculation produced a mild fever and some short-lived discomfort in the boy but no other signs of infection. Two and a half weeks later, Jenner injected the boy with smallpox material, and the boy remained healthy. Jenner repeated the smallpox challenge 20 times, and the boy never showed any signs of smallpox infection.
Jenner then successfully repeated his experiment on 23 additional people. Jenner's findings were published and quickly spread among physicians in Europe and America. By 1801, physicians all across Europe and even a few in North America started to use Jenner's vaccination procedure. Increased use of the vaccine and a concerted effort by the World Health Organization more than a century later to rid the world of smallpox through vaccination, resulted in complete eradication of smallpox by 1977.
The smallpox vaccine is an example of a live vaccine, which uses live, infectious virus as the actual vaccine. Live vaccines are usually just weaker, less dangerous forms of the disease-causing virus. These so called live, attenuated vaccines are created in the laboratory under conditions that promote less harmful characteristics of a virus. In the case of the smallpox vaccine, nature provided a much less dangerous alternative, which could still impart immunity to smallpox. A rather fortunate coincidence for humankind and modern medicine, especially given how deadly and infectious smallpox was.
Some vaccines called inactivated vaccines are created by killing or inactivating the pathogen and then using it as the vaccine itself. An example is the pertussis vaccine that protects against whooping cough. Inactivated vaccines are easier to create than live, attenuated vaccines and don't carry the risk of mutating back into a disease-causing form. However, because this type of vaccine isn't active and therefore, doesn't infect the body's cells, the immune response that it produces is not as strong as an immune response to a virus that is actively infecting cells. The resulting immunity is also not as strong and fades more quickly over time because fewer memory cells are produced. Therefore, inactivated vaccines must be administered more than once, sometimes three or more times to create a strong enough immunity to the infectious agent. Each injection of the vaccine after the initial injection is called a booster shot, and with each additional shot, more memory cells are produced and the body's immunity is strengthened.
In certain types of bacterial infections, the disease is caused by a toxin produced by the organism. In most of these situations, it has been found that the toxin can be inactivated with formalin and used as a toxoid vaccine. An example is the tetanus vaccine. An immune system primed against a toxin will be able to identify and destroy the toxin before it can do too much harm, especially if booster shots are given.
Recombinant subunit vaccines are vaccines that use recombinant DNA technology to produce high quantities of antigens that elicit an immune response. With the rapid advances in recombinant DNA technology, these types of vaccines have become easy to create and produce. The real challenge with these vaccines is finding the antigens that will produce an immune response strong enough to create immunity. A recombinant subunit vaccine for hepatitis B is now commercially available.
And finally, DNA vaccines hold lots of promise. There aren't any approved DNA vaccines on the market yet, but DNA has the potential to combine the best attributes of other vaccines. DNA is easy to produce, manipulate, transport, store and administer. In addition, if the DNA is incorporated into the genomes of some of the body's cells, it could mimic a viral infection without having the danger of introducing a virus that could revert back to a dangerous form.
In 1928, bacteriologist Alexander Fleming made one of the most important discoveries in modern medicine while cleaning up some old bacterial cultures. On a plate of Staphylococcus aureus, a contaminating colony of mold was growing on the plate. Fleming was going to clean out the dish with Lysol, but before he did, he noticed something strange about the plate. There was a ring around the mold where no bacteria were growing. Being a keen observer and a bacteriologist who was looking for a human-safe drug that could combat bacteria, Fleming recognized the potential in this contaminating mold for an antibacterial agent. The mold was identified as a Penicillium mold, so he named the antibacterial agent that it produced Penicillin, even though he wasn't able to isolate it himself.
Not to worry, though, because other scientists eventually purified Penicillin, developed methods to mass produce it and figured out how to use it to combat many types of bacterial infections, including Staphylococcus aureus, diphtheria, gangrene, pneumonia, syphilis and tuberculosis. Penicillin was the first discovered antibiotic or substance with antimicrobial properties that can be tolerated by humans that was effective against bacteria. Antibiotics are fundamentally different from vaccines because they do not stimulate the immune system. Instead, they directly kill or inhibit the growth of microorganisms. The first major application of Penicillin was in the mid-1940s by Allied forces to treat life-threatening bacterial infections of Allied soldiers and is credited with saving hundreds of thousands of lives during the war. Penicillin was extremely effective against many types of bacteria, but there were lots of diseases that it was not effective against as well.
Soon other molecules were discovered that had antimicrobial properties as well, each with a different set of diseases that they could combat. Some were more effective than others, and some had undesirable side effects in patients. The earliest antibiotics, like Penicillin, were molecules isolated from natural sources, like molds and plants. However, as chemical techniques became more advanced, chemists started altering these molecules and formed new forms of antibiotics. Eventually, chemists started making completely synthetic molecules and tested them by the thousands for antimicrobial properties. Today, there are over a hundred known antibiotics, each with their own characteristics and microbial targets, and even as we speak, drug companies are still working to develop new types of antibiotics.
So, in review, between 1850 and today, the average life expectancy in the U.S. has nearly doubled, and most of that can be attributed to the control and prevention of infectious diseases through improved sanitation, vaccinations and antibiotics.
Improved sanitation may seem like an obvious way to limit disease, however, before Louis Pasteur essentially proved that infectious diseases are caused by microscopic organisms that must be transferred from one host to another in order to spread, otherwise known as the germ theory of disease, sanitation was often ignored as a factor in the spread of disease.
Strangely enough, even before the germ theory of disease was widely accepted by the scientific community, vaccinations had already been discovered and effectively used against the spread of smallpox. The smallpox vaccine has been so successful that the disease has now been completely eradicated. No other infectious disease has been eradicated, but vaccines have been able to prevent large scale epidemics of many other infectious diseases that were quite common before 1800.
And finally, antibiotics were the third key breakthrough that has helped cure millions of various types of infectious diseases. Without antibiotics, diseases like strep throat, syphilis, gangrene and staph infections would still be deadly killers.
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Back To CourseCLEP Biology: Study Guide & Test Prep
24 chapters | 233 lessons | 22 flashcard sets