The Black Death: The very name evokes horrifying images (Figure 11.5). This pandemic outbreak of bubonic plague, which peaked in Europe around 1350, killed an estimated one-third of Europe’s population, convincing many people at the time that they were witnessing the end of the world. Bubonic plague is only one of the devastating pandemics humans have experienced; smallpox, infant diarrhea, malaria, tuberculosis, and cholera have killed millions of people.

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At the time, it was thought that the Black Death, and disease in general—a condition in which normal biological function is impaired by bacteria, viruses, parasites, improper diet, or pollutants—were caused by foul or noxious air. It was not until 500 years later, in the late 19th century, that one of the key scientific breakthroughs in the fight against infectious disease revealed that infectious bacteria cause many diseases, including bubonic plague.

Bacteria are not the only pathogens (organisms that produce illness). Viruses and parasites also produce disease in humans and other organisms. Although we now understand how most diseases are transmitted, infectious disease has remained the leading cause of death due to environmental hazard over the last century. Let’s consider the three classes of pathogens—namely, bacteria, viruses, and parasites—beginning with bacteria.

Bacteria (singular bacterium) existed billions of years before the first multicellular organisms developed. Most bacteria are single-celled and very small—a fraction of the size of human body cells—with a very simple structure (Figure 11.6). Because of their small size, bacteria remained invisible to humans until the invention of the first microscopes in the late 1600s, and it would be more than a century before it was discovered that bacteria cause diseases.

Bacteria live everywhere—from down in the deepest mines to a piece of dust floating in the atmosphere—and are essential to the functioning of the biosphere. Most bacteria are benign—that is, they do not cause disease—and many are, in fact, essential to human health. The body of an adult human includes more than 100 trillion cells, but only about 10 trillion of them are human cells. The remaining 90% of cells in and on the human body, your body, are bacteria; these 90 trillion bacterial cells make up only a small proportion of the human body’s mass, but nearly all of them are beneficial. For instance, bacteria in the intestine help to digest food, and bacteria on the skin and in the nose help prevent harmful bacteria from invading those areas.

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It is the harmful bacteria that cause bacterial diseases, but they normally act with an environmental agent. An environmental agent can be as simple as a sharp object that penetrates the skin and allows bacteria to invade the tissues beneath; or it can be as complex as those associated with the cholera bacterium, which adjusts its lethality in response to the pollution of water sources by human waste.

The majority of disease-causing bacteria affect the host organism they invade by producing substances that damage cells in the host’s body. Bacteria produce these toxins in two ways. Exotoxins are proteins secreted by bacteria into their surrounding environment. Some of these exotoxins, such as botulin, which is responsible for the food poisoning known as botulism, are among the most toxic natural substances known. Cholera, an often fatal disease of the small intestine, is the result of an exotoxin produced by the bacterium Vibrio cholerae. By contrast, endotoxins are part of the cell membranes of some bacteria, such as Salmonella, which is responsible for another type of food poisoning. Endotoxins are released when bacterial cells die and disintegrate.

Other bacterial diseases, such as tuberculosis, are the result not of toxins but of bacteria growing in the tissues of their host and competing with the host’s cells for nutrients.

Viruses are responsible for numerous health problems, ranging from relatively mild diseases, such as the common cold, to some of the most destructive diseases, including smallpox, rabies, Ebola, and AIDS (acquired immune deficiency syndrome), which is caused by HIV (human immunodeficiency virus).

Viruses, like bacteria, are pervasive in the environment. Although a virus is a microscopic pathogen like a bacterium, the two are quite different biologically. Viruses consist simply of genetic material encased in protein; they do not have the cell structures and biochemical pathways that allow bacteria to carry out all the processes necessary for life (Figure 11.7a). A virus can complete its own life cycle only by invading and taking over the cellular systems of animals, plants, or bacteria (Figure 11.7b). Viruses essentially highjack the machinery of a cell and use it to produce copies of the virus instead of carrying out the normal functions of the cell. When cells stop their normal processes, the symptoms of viral disease appear.

The damage that some viruses cause can be repaired. Viruses kill the cells they infect, but if healthy cells multiply, they replace the dead cells. Some cells, such as nerve cells, are not actively replaced, however, and the damage to the body can be permanent. Polio is an example of a viral disease that attacks nerve cells with crippling effects. HIV attacks cells of the human immune system, reducing the capacity of the body to resist infectious diseases. The Ebola virus, which is endemic to several African countries, also attacks the immune system. It causes fever and widespread inflammation, which damages the liver, intestine, and blood vessels and potentially leads to bleeding from the eyes and nose.

One of the most common types of viruses that affect humans is the influenza virus. Most of us have had the flu at one time or another and have suffered from its symptoms: fever, cough, congestion of the lungs, sore throat, headache, sore muscles, and fatigue. Though most people recover from the flu within a week or two, the disease still takes a substantial toll on populations, especially among the elderly and the chronically ill. In the United States each year, approximately 100,000 victims of seasonal flu are hospitalized and there are about 20,000 flu-related deaths. Around the world, seasonal flu causes 3 to 5 million cases of severe illness and a quarter to half a million deaths annually.

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“It’s flu season, be sure to get your shot!”—it’s a familiar refrain each year. Annual vaccinations are necessary for protection because viral genetic material, in this case that of the influenza virus, can change. In other words, viruses evolve. One of the most changeable of the viruses is HIV, which makes combating AIDS much more difficult.

Parasitism is another major source of disease in human populations. A parasite is an organism that lives in or on another organism, called the host (Figure 11.8). Parasitism is a particular type of ecological relationship similar to that between predators and prey (see Chapter 3, page 71), in which the host is harmed by the parasite and the parasite extracts benefits from the host, such as food, protection, dispersal of offspring, and so forth.

Parasitism is a very successful way of life. Because every species of animal, including all insects and other arthropods, is a host to at least one species of parasitic roundworm, and because most animal species serve as hosts for multiple species of roundworms, there are probably more species of roundworms than all other kinds of animals combined. However, they remain understudied, so we don’t know for sure.

Humans serve as hosts to at least several hundred species of parasites that live on us and in us. For instance, outbreaks of head lice plague schools every year, and infestations of bed bugs are becoming a problem for travelers, even those who stay in expensive hotels. Both lice and bed bugs are external parasites that suck blood, as are mosquitoes and other biting insects. A variety of worm-shaped organisms are internal parasites of humans. Tapeworms live in the intestine. The largest human parasite, a tapeworm that can reach a length of more than 30 feet and has a life span of 20 years, can infect people eating raw or undercooked fish. Hookworms also live in the intestine, but they enter the body by penetrating the skin on the soles of the feet.

Many parasites do not cause a disease, but a heavy infestation of blood-sucking parasites can produce anemia (an abnormally low content of hemoglobin in the blood) or reduce the levels of important vitamins and minerals in the tissues. Some parasites transmit bacteria or viruses: Some ticks, for example, transmit the bacterium that causes Lyme disease, and mosquitoes transmit disease-causing viruses, such as the West Nile virus and the eastern equine encephalitis virus.

Seven of the top 10 diseases targeted by the World Health Organization’s Special Programme for Research and Training in Tropical Diseases are parasitic diseases. Tropical parasitic diseases take a terrible toll on human populations, infecting hundreds of millions of people each year. They are responsible for more than a million deaths annually and, in some cases, are spreading rapidly. The range of some tropical diseases, such as dengue fever, is expanding as global climate change allows the mosquitoes that carry the disease to spread northward.

Pathogens are found virtually everywhere in the environment. Bacteria and viruses travel in air, in water, and in the bodies of organisms. They can enter through open wounds, via the lungs, and via the intestine.

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However, pathogens must enter a host’s body before they can cause an infection, and hosts are well protected. The dense populations of benign bacteria on the skin and in the nose, mouth, and intestine form the first line of defense against infection by pathogenic bacteria. By occupying every site in the body that is suitable for bacterial growth, these harmless bacteria make it difficult for invading pathogenic bacteria to find a place to live. But, by invading in overwhelming numbers, disease-causing bacteria can break through a host’s defensive wall of protective bacteria. Hundreds, thousands, or even millions of bacterial cells are needed to initiate an infection by most pathogenic bacteria.

Parasites are often transmitted indirectly, through secondary organisms called vectors. Some of the most significant vectors are biting insects such as mosquitoes and flies. The Plasmodium protozoan, which causes malaria, is carried by mosquitoes and is injected into its animal host when a biting mosquito releases saliva to prevent blood from clotting (Figure 11.9).

Schistosomiasis is a parasitic disease common in many parts of the world, especially in areas with poor sanitation. The organism that causes schistosomiasis reproduces in humans. Its eggs are excreted with human urine and feces; when the eggs enter a body of water, they hatch into free-swimming organisms that enter the bodies of snails. There, they pass through more stages and are released into the water as yet another stage, called cercariae. The cercariae penetrate the skin of humans who enter the water and travel through the body of the human host, eventually lodging in the intestine, where they mature and produce another generation of eggs.

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