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Infectious Disease Diseases are caused by pathogens that invade a host body. Pathogens need a host in order to survive and multiply. Some examples of pathogens are bacteria, viruses, and fungi. They can spread through direct body contact, body fluids, and contact with an object that an infected person has touched (some viruses, like the common cold virus, can exist outside the body for a brief period before they get passed on to another host). Tuberculosis is also an infectious disease. Victims of tuberculosis cough up blood from their lungs. Treatment and vaccines for tuberculosis exist, and this disease has been almost elim- inated in some parts of the world. However, the total number of people in the world infected with tuberculosis keeps growing. Noninfectious Disease If the disease cannot spread from person to person, then it is considered noninfectious. Two examples of noninfectious diseases are cancer and heart disease. Here are some characteristics of noninfectious diseases: ■ They do not transfer from person to person. ■ They are not caused by viruses, bacteria, or fungi. ■ They are sometimes hereditary—meaning that they are associated with genes and run in families. Noninfectious diseases can be classified further: ■ Hereditary diseases. Hereditary diseases are caused by genetic disorders passed down from previous generations. Since they are inherited, they are more difficult to treat because they are a part of the body’s genetic makeup. ■ Age-related diseases. Some diseases will start to develop as the body gets older. As the body grows old, it does not work as efficiently to battle rou- tine diseases and degenerative diseases such as Alzheimer’s disease—which causes mild to severe memory loss or distortion, forgetfulness, anxiety, and aggressive behavior. ■ Environmentally induced diseases. An environment that has been polluted with toxins and haz- ardous waste can affect the population living in or around it. Radiation from toxic waste can cause cancer. Exposure to asbestos can lead to serious lung problems. Staying healthy by caring for the body is important in fighting and preventing disease. Poor hygiene and unhealthy living conditions are invitations for disease. Here are a few tips to stay healthy: ■ Eat a nutritious diet. ■ Keep your hands and body clean. ■ Exercise regularly. ■ Reduce stress. ■ Don’t smoke. ■ Don’t drink excessively. It is also important to feel good about yourself.A pos- itive view of who you are and what you look like can help reduce stress considerably. Looking for Symptoms Before diagnosing a patient with a disease, a doctor looks for the telltale symptoms. Every disease has specific symptoms that cause different reactions in the body. Some of the more common symptoms are fever, nausea, and pain. A doctor is trained to look for these symptoms to give a correct diagnosis and issue proper treatment. Blood tests and X-rays are special methods used to diag- nose some diseases. Epidemics An epidemic is a disease that has infected a considerable portion of the population and that continues to spread rapidly. Epidemics can occur when there is no medicine for the disease, when diseases develop a resistance to medicine and drugs, or when environmental conditions are favorable for a specific type of disease. For example, cancer is rampant in areas with toxic chemicals and high levels of radiation. Autoimmune deficiency syndrome, or AIDS, which is caused by the HIV virus, is an epidemic that is killing millions of people worldwide. HIV is spread through sexual contact and through contact with the blood of an infected person. Natural and Medical Defenses Humans and most other living beings have a natural built-in disease-fighting mechanism known as the immune system. The immune system is composed of cells, molecules, and organs that defend the body against pathogens. The immune system is responsible for find- ing the pathogen in the body and killing it, rendering it harmless, or expelling it from the body. – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 250 The development and use of vaccines and antibiotics has added to our defenses against diseases. Not only have advances in medicine found ways to fight disease from inside the body, but methods have also been developed to prevent the onset of disease. ANTIBIOTICS Antibiotics are chemicals that kill bacteria without harming our own cells. Some antibiotics, such as peni- cillin, kill bacteria by preventing it from synthesizing a cell wall. Other antibiotics interfere with bacterial growth by disrupting their genes or protein production. Bacteria can become resistant to antibiotics—there are strands of bacteria that are resistant to every known antibiotic. RESISTANCE In every population, a small number of bacteria natu- rally have genes that make them resistant to antibiotics. With increased exposure to antibiotics, a normal population of bacteria, having a few resistant individuals, becomes resistant on average. This is a result of natural selection. Those bacteria that survive are resistant. Their offspring is also resistant, and as a result, the whole population becomes resistant. Some resistance enables bacteria to survive in the presence of an antibiotic. Another kind of resistance enables the bacteria to actually destroy the antibiotic. This kind of resistance is most dangerous. For example, someone who took antibiotics for treating acne could accumulate bacteria capable of destroying the antibiotic. If that same person became infected with a serious disease that is treated with the same antibiotic, the resistant bacteria could destroy the antibiotic before it was able to act on the disease. Community and Public Health People are dying from diseases in many parts of the world where clean water is scarce and living conditions are poor. Educating people on the importance of personal hygiene, cleanliness, and sanitation is the key to preventing disease in these populations. A clean, healthy environment will ensure better health and safety.  Population Growth and Control The human population growth rate was increasing rela- tively slowly up until 1,000 years ago. Before the inven- tion of vaccines and antibiotics that prevented deadly infectious diseases, and before humans developed plumbing and sewage treatment plants to ensure safe, clean drinking water, factors such as the spread of diseases increased death rate. Lack of food supply and intol- erance for living in extremely hot or extremely cold environments are also examples of limiting factors that control population growth. By the early 1800s, the world population reached 1 billion. It took approximately 2.5 million years for humans to reach this mark. But now, only 200 years later, the world population has reached 6 billion. From 1850 to 1930, a period of less than 100 years, the estimated world population doubled. In 1975, less than 50 years later, the world population doubled again to reach 4 billion. Then, only 12 years later, it reached 5 billion. It is estimated that by 2050, the world population will reach 10 billion. When a couple has two children, each child replaces one of the parents, and in theory, the population should stay the same. However, due to increased life expectancy, several generations of people are alive at the same time. It is estimated that even if everyone from now on had only one or two children, the population would continue to grow for about 50 years. The reason for this is that most of the world population is young and has yet to reproduce. In a way, the population has a momentum and its growth cannot stop immediately, in much the Human Population Growth 12 10 8 6 4 2 0 1800 1850 1900 1950 2000 2050 Population (in billions) Year – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 251 same way that you can’t instantaneously stop a car that is running at 70 miles per hour. Coming to a stop takes time. Even if everyone in the world from this moment on started having no more than two children, the population would continue to grow for about another 50 years. How Did This Happen? So how did the human population grow so much and so rapidly? One of the main reasons is that many limiting factors to human population growth have been elimi- nated. Here are some explanations: Advances in medicine and healthcare enabled the development of: ■ vaccines to prevent the spread of infectious diseases ■ antibiotics to cure common illnesses ■ therapies to treat patients with noninfectious diseases such as cancer Advances in technology enabled humans to: ■ expand into new habitats ■ live in places with extreme climate conditions ■ develop sanitation and sewage-disposal systems Advances in science enabled humans to: ■ increase food supply and improve living conditions ■ reduce deaths from natural disasters and other hazards ■ use the Earth’s natural resources such as fossil fuels Since people have learned to overcome some of the limiting factors that prevented human growth and survival, the death rate has steadily decreased. Because of the increase in production of food supply and other resources, the infant death rate has also decreased. What Does This Mean for Our Future? So what will happen if the human population continues to grow at this rate? The result is overpopulation. Over- population occurs when there are too many individuals in a given area, so that the resources are depleted faster than they can be replaced. Overpopulation is not the same as overcrowding, which is another consequence of steady population growth. Overcrowding occurs when there are too many individuals living in an area—to the point where most of the individuals in the population live in substandard or poor conditions because of lack of work and lack of living space. Mexico City, Istanbul, China, and India are some examples of places in the world experiencing overcrowding. How Will Overpopulation Affect Us? Overpopulation can cause serious damage to our way of life as well as our environment. Here are just some effects of overpopulation. ■ Hunger and starvation. Technology has enabled us to develop ways to improve food production and agriculture. However, the rate of food production increase—at this moment—is not keep- ing up with the rate of population growth. In other words, the amount of mouths to feed is increasing faster than our ability to feed them. The uneven distribution of food, rather than the lack of food, however, is causing most of the hunger problems. While huge amounts of food are being thrown away in some parts of the world, people in other parts of the world are starving to death. ■ Depletion of our natural resources. Some resources are depleted faster than they are replenished. Our oil and coal supplies, for example, take millions of years to replenish, and given the con- sumption rate, they will eventually run out. ■ Ozone layer and global warming. Ozone is a very reactive molecule, made of three oxygen atoms. At about ten to thirty miles above the Earth, a layer of ozone molecules absorbs ultravi- olet light (UV) emitted by the sun and shields living things from potentially dangerous amounts of this radiation. UV light can increase the amount of mutations in DNA. Some biologists believe that too much UV light has driven some species – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 252 of frogs to extinction. In humans, excess UV light is a major cause of higher rates of skin cancer. About 20 years ago, scientists began to document a thinning of the ozone layer, especially over Antarctica, where the ozone hole is larger than the size of North America. The depletion of the ozone layer is due largely to deforestation (to make room for houses, roads, and buildings) and chemicals, such as chlorofluorocarbons (CFCs), that are being released into the atmosphere. CFCs are small molecules used as coolant in refrigera- tors and air conditioners and as propellants in some spray cans. The evidence that CFCs are destroying the ozone layer has become so clear that CFC producers have agreed to replace these compounds with others. ■ Effect on biodiversity. Overpopulation has a pro- found effect on biodiversity. In order to make room for ourselves, our houses, factories, and shopping centers, and to come by food and energy sources, we have disrupted natural animal and plant habitats. One way in which humans contribute to the extinction of species is by frag- menting their habitats—splitting them into several smaller habitats. This decreases the genetic diversity and structure of a habitat, which leads to inbreeding, reduced reproduction, and small population size. A small, inbred population is more likely to become extinct. Extinction of one species can lead to extinction of another that depends on the first for food. ■ Pollution. Waste is produced faster than it can be dispersed or biodegraded. This causes the buildup of contaminants that can affect our water, soil, and air. Noise can also contaminate environments, especially in cities. This phenome- non is called noise pollution. Light pollution is another problem. Very few stars are visible from most cities, even on a clear night, because there is too much artificial light around. Images taken of North America at night show a series of bright spots throughout the continent. Traveling by plane at night makes the overwhelming amount of artificial light produced by humans very noticeable. Research suggests that light at night can affect the production of certain hormones and, in return, increase some health risks. In addition, excess light may be harmful to animals as well. Much of the problem can be solved by turning on only the lights that are absolutely necessary for safety reasons; making them only as bright as they need to be; pointing them toward the ground, not the sky; and shielding them to prevent scattering. Implementing these kinds of solutions will also help conserve resources by saving electricity.  Natural Resources Humans depend on resources to sustain life. A good part of our everyday resources come directly from the environment. These are called natural resources—resources provided by nature. Air, water, sunlight, topsoil, and the various plant and animal life known as biodiversity are examples of Earth’s natural resources. There are two kinds of natural resources: renewable and nonrenewable. 1. Renewable resources are those that can be replaced or replenished over a short period of time. Plants and crops are examples of resources that, with proper agriculture, are replenishable. 2. Nonrenewable resources are those that cannot be replaced or that take many years to replenish. Fossil fuels such as oil and coal are examples of nonrenewable resources. Depletion of Natural Resources Currently, many of our nonrenewable resources are in danger of being depleted. Water, topsoil, and energy are some of the essential resources that are in short supply. ■ Wa t e r. Water is necessary for agriculture, but it is currently the resource in shortest supply. Some parts of Africa and the Middle East are experiencing mass starvation as a result of drought, or water shortage. Availability of drinking water, free of chemical waste, is also decreasing. ■ Topsoil. Fertile topsoil takes hundreds, maybe even thousands of years to replace. Human activi- ties have already caused degradation of some of Earth’s fertile topsoil, and as a result, the degraded topsoil is no longer able to sustain agriculture. – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 253 ■ Energy. Most of our energy resources come from fossil fuels such as oil and coal. They are used for heat, electricity, and gasoline. Fossil fuels are decreasing in supply worldwide because they are being used faster than they are being produced. Reuse, Reduce, and Recycle: Preserving Our Natural Resources So how do we prevent our natural resources from deplet- ing? There are several ways to help protect our natural resources. CONSERVE It is important that we all learn to conserve our natural resources. To conserve is to limit or control the use of natural resources, especially nonrenewable resources. While big industries are most responsible for energy use and pollution, small consumers (like you), in the com- pany of six billion other small consumers, can have a notable effect on the use and preservation of natural resources. So: ■ If you are the last one to leave a room, turn off the lights. This will save electricity. ■ When you brush your teeth, do you leave the water running? If you shut the water off while you brush, you are conserving water. ■ Walking short distances instead of driving will save fuel and limit air pollution. RECYCLE One way to protect our environment is by recycling— reusing solid waste as is or breaking it down to make new products. ■ Old newspaper and cardboard can be shredded up and recycled to make new paper. ■ Glass bottles can be melted down and used to make new bottles. These are examples of resource recovery, where the raw materials are extracted to make new ones. Another form of recycling is reuse. If you have an old car, sell or donate it rather than discarding it. In this way, the car is recycled. Much of our solid waste can be recycled. By recycling, we are decreasing the demand for use of more natural resources and decreasing the amount of space needed for waste disposal. Glass, paper, metal, and plastics are a few examples. If we recycled all our paper garbage, it would save thousands of trees every year from being chopped down to make paper. Recycling aluminum and other metals is more energy efficient than creating them from metal ores. PROTECT B IODIVERSITY Protecting biodiversity—the various plant and animal life on Earth—means protecting our sources of food, water, clean air, and fertile topsoil. Extinction, or the dying off of species of plants and animals, damages biodiversity. Humans play a big part in causing the extinction of essential plant and animal life by: ■ interfering with and destroying natural habitats ■ polluting the air and water that feed plants and animals ■ using illegal methods (e.g., explosives) for fishing ■ killing already endangered species COME UP WITH BETTER SOLUTIONS Another option is to come up with better solutions— new ways of using or obtaining energy, developing more efficient processes, and better designs. For example, electric cars are beginning to show up in major cities like San Francisco and Los Angeles. Usually available for rent to cruise the city in style, these little innovations are starting to make it to the consumer mar- ket. If you have an AC power outlet in your garage, you are all set to own an electric car. The benefits of owning an electric car are easy to guess. They are quiet and don’t emit toxic chemicals that deplete the ozone layer. They also conserve natural resources needed to make gasoline.  Science and Technology in Local, National, and Global Challenges Science affects the way we live, work, act, and play. Our technological abilities have also given us the ability to confront certain global challenges. But we need to con- sider where our technological abilities lead us and make sure that our own might doesn’t destroy us. By having a basic science education, we are taking the first step in preventing this from happening. – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 254 War Along with advances in technology came a different kind of warfare—mass destruction and complete disregard for the environment. To end World War II and test a new weapon, the United States dropped two atomic bombs on Japan, instantly ending countless lives. Chemical and biological weapons, and cluster bombs containing depleted uranium, present another danger. All these weapons affect not only the humans involved in wars now, but future generations, and plant and animal life. Solar Power Solar power refers to the conversion of solar energy to another, more useful form. Sunlight can be harnessed and collected in special greenhouses. Photosensitive cells can produce electricity when sunlight hits them. The sun produces about ten times the energy fossil fuels create each year. Many scientists are convinced that this form of energy will one day replace ordinary fossil fuels. Cur- rently, one reason that we still do not see solar-powered cars and houses is because fossil fuels are cheaper to col- lect and use. But technology is slowly catching up—solar plants are now being constructed in some parts of the United States. Scientists are hopeful that these new plants will be able to produce enough energy to power our cities in the future. Genetic Engineering One of the fastest growing fields in science, and also pos- sibly the most controversial, genetic engineering, has been making headline news. The first thing that comes to mind is cloning. But there is more to genetic engineering than that. Genetic engineering is used to produce everyday products such as fruits, grains, plants, and even animals like fish. This might be a bit pointless, you might say. Certainly, we have had fruits, plants, and animals before. Why do we have to genetically engineer these products? We do not make these products from scratch. Genetic engineering allows us to modify the product to bring out certain qualities or to embed qualities that the product would not normally have. For example, Florida oranges grow best in Florida because oranges prefer lots of sun and warm temperatures. Genetic engineering can modify the trees so that the oranges can grow in colder cli- mates, like further north. While making an orange tree that can grow anywhere seems like a good idea, we must look at the flip side and examine other projects. What effect would an orange tree in Alaska have on other plant and animal life in Alaska? In China, scientists concerned with overpopulation and hunger developed a strain of rice that will grow twice as fast as normal rice. This means that more food can be produced faster. Unfortunately, the faster-growing rice has half the nutrients of normal rice. Is this a step up? Now there is more rice available for the population, but it is less nutritious than natural rice.  Environmental Quality Many factors contribute to environmental quality. Pol- lution, the introduction of substances that affect or harm the environment, is one of the biggest environmental concerns scientists face today. There are many different forms of pollution. Some are natural, like volcanic eruptions. Humans, however, cause most other forms of pollution. Air Pollution Air is polluted by the introduction of harmful contaminants into the atmosphere. In and around big cities, smoke produced from factories and car emissions is called smog. Smog in the atmosphere can cause acid rain. Recently, people with allergic reactions to smog have found the need to catch the smog alerts commonly read with the weather reports. In addition to causing allergies, smog has been known to cause numerous health problems, damage habitats, and disrupt ecosystems. Water Pollution Many companies dispose their waste by pumping it into rivers, causing pollution in our water systems. Sewage and pesticides are also factors that contribute to water pollution. About one in three rivers in the United States is polluted. This presents serious problems to all life that depends on clean water for survival. Oceans also get polluted. Garbage dumping, oil spills, and contaminated rivers are the biggest con- tributing polluters for our oceans. This can be devastat- ing for countries that depend heavily on fishing for food. In 1989, the oil tanker Exxon Valdez smashed into some rocks and spilled 260,000 barrels of oil in Alaska. The – PERSONAL AND SOCIAL PERSPECTIVES IN SCIENCE– 255