Environmental Problems, Their Causes, and Sustainability 1

19-4 BIOLOGICAL HAZARDS:DISEASE IN DEVELOPED ANDDEVELOPING COUNTRIESWhat Are Nontransmissible and TransmissibleDiseases? To Spread or Not to SpreadDiseases not caused by living organisms do notspread from one person to another, and thosecaused by living organisms such as bacteria andviruses can spread from person to person.A nontransmissible disease is caused by somethingother than a living organism and does not spread fromone person to another. Such diseases tend to developslowly and have multiple causes. Examples are cardiovascular(heart and blood vessel) disorders, most cancers,diabetes, asthma, emphysema, and malnutrition.A transmissible disease is caused by a living organismand can spread from one person to another. Infectiousagents or pathogens (such as a bacterium, virus,protozoa, or parasite; Figure 19-8) cause such diseases.These agents are spread by air, water, food, and bodyfluids, and by some insects (such as mosquitoes; Figure19-9, p. 420) and other nonhuman carriers. All suchpathways are called vectors.Typically, a bacterium is a one-celled microorganismthat can replicate (clone) itself by simple cell division.A virus is a microscopic, noncellular infectiousagent. Its DNA or RNA contains instructions for makingmore viruses, but it has no apparatus to do this. Toreplicate, a virus must invade a host cell and take overthe cell’s DNA to create a factory for producing moreviruses (Figure 19-10, p. 421). A parasite is an organismthat feeds off another organism (p. 154). Protozoans are adiverse assortment of microscopic or near-microscopicorganisms that live as single cells or in simple colonies.Examples are Giardia lamblia that causes giardiasis, agastrointestinal disease transmitted by water, and severalspecies of Plasmodium that transmit malaria.According to the World Health Organization,about 30% of all deaths per year are caused by nontransmissiblecardiovascular disease, 26% by transmissibleinfectious disease (Figure 19-11, p. 421), and 12%by nontransmissible cancers.As a country industrializes, it usually makes anepidemiological transition in which deaths from the infectiousdiseases of childhood decrease and those fromthe chronic diseases of adulthood (heart disease andstroke, cancer, and respiratory conditions) increase.Good news. Since 1900, and especially since 1950,the incidence of infectious diseases and the death ratesfrom such diseases have been greatly reduced. Thishas been done mostly by a combination of betterhealth care, using antibiotics to treat infectious diseasecaused by bacteria, and developing vaccines to preventthe spread of some infectious viral diseases.Bad news. Many disease-carrying bacteria have developedgenetic immunity to widely used antibiotics(Case Study, below). Also, many disease-transmittingspecies of insects such as mosquitoes have become immuneto widely used pesticides that once helped controltheir populations.Case Study: Are We Losing Ground in OurStruggle against Infectious Bacteria? GrowingGerm Resistance to AntibioticsRapidly producing infectious bacteria can undergonatural selection and become genetically resistantto widely used antibiotics.We may be falling behind in our efforts to preventinfectious bacterial diseases because of the astoundingreproductive rate of bacteria, which can produce16,777,216 offspring in 24 hours. Their high reproductiverate allows them to become genetically resistant toan increasing number of antibiotics through naturalselection. They can also transfer such resistance tononresistant bacteria.Other factors play a role in the potentially seriousrise in the incidence of some infectious bacterial diseases—suchas tuberculosis (Case Study, p. 421)—oncecontrolled by antibiotics. One is that harmful bacteriaare spread around the globe by human travel and thetrade of goods. Another is that overuse of pesticidesVirusesHepatitis BHIV(AIDS)SmallpoxBacteriaVibrio cholerae(cholera)Myobacteriumtuberculosis(tuberculosis)ProtozoaEbolaOn this scale, a human hair would be 6 meters (20 feet) wideTreponema pallidum (syphilis)Plasmodium(malaria)1 micrometer 6 micrometers10 micrometersFigure 19-8 Examples of pathogens or agents that can cause transmissible diseases. A micrometer is onemillionthof a meter.http://biology.brookscole.com/miller14419