Unit Four. The Evolution and Diversity of Life


16. Prokaryotes: The First Single-Celled Creatures


16.10. Disease Viruses


Humans have known and feared diseases caused by viruses for thousands of years. Among the diseases that viruses cause (table 16.3) are AIDS, influenza, yellow fever, polio, chicken pox, measles, herpes, infectious hepatitis, and smallpox, as well as many other diseases not as well known.




The Origin of Viral Diseases

Sometimes viruses that originate in one organism pass to another, causing a disease in the new host. New pathogens arising in this way, called emerging viruses, represent a greater threat today than in the past, as air travel and world trade in animals allow infected individuals and animals to move about the world quickly, spreading an infection.



Influenza. Perhaps the most lethal virus in human history has been the influenza virus. Between 40 and 100 million worldwide died of flu within 19 months in 1918 and 1919—an astonishing number. The natural reservoir of influenza virus is in ducks, chickens, and pigs in central Asia. Major flu pandemics (that is, worldwide epidemics) have arisen in Asian ducks through recombination within multiple infected individuals, putting together novel combinations of virus surface proteins unrecognizable by human immune defenses. The Asian flu of 1957 killed over 100,000 Americans. The Hong Kong flu of 1968 infected 50 million people in the United States alone, of which 70,000 died. The swine flu epidemic of 2009, discussed on page 351, killed thousands of American children.



AIDS (HIV, human immu- nodeficieny virus). The virus that causes AIDS first entered humans from chimpanzees somewhere in Central Africa, probably between 1910 and 1950. The chimpanzee virus, called simian immunodeficiency virus, or SIV, mutates rapidly, at a rate of 1% a year, and after it entered humans it continued to do so, soon becoming what we now know as HIV and spreading to human populations worldwide, mostly through sexual contact with an infected person. AIDS was first reported as a disease in 1981. In the following 30 years, 25 million people have died of AIDS, and 33 million more are currently infected. Where did chimpanzees acquire SIV? SIV viruses are rampant in African monkeys, and chimpanzees eat monkeys. Study of the nucleotide sequences of monkey SIVs reveal that one end of the chimp virus RNA closely resembles the SIV found in red-capped mangabey monkeys, while the other end resembles the virus from the greater spot-nosed monkey. It thus seems certain that chimpanzees acquired SIV from monkeys they ate.



Ebola virus. Among the most lethal of emerging viruses are a collection of filamentous viruses arising in Central Africa that attack human connective tissue. With lethality rates in excess of 50%, these so-called filoviruses cause some of the most lethal infectious diseases known. One, Ebola virus, has exhibited lethality rates in excess of 90% in isolated outbreaks in Central Africa. Luckily, victims die too fast to spread the disease very far. Researchers have reported evidence implicating fruit bats as Ebola hosts. These large bats are eaten for food everywhere in Central Africa that outbreaks have occurred.



Hantavirus. A sudden outbreak of a highly fatal hemorrhagic infection in the southwestern United States in 1993 was soon attributed to a strain of hantavirus, an RNA virus associated with rodents. This strain was eventually traced to deer mice. The deer mouse hantavirus is transmitted to humans through fecal contamination in areas of human habitation. Control of deer mouse populations has limited the disease.



SARS. A recently emerged strain of coronavirus was responsible for a worldwide outbreak in 2003 of severe H acute respiratory syndrome (SARS), a respiratory infection with pneumonia-like symptoms that in over 8% of cases is fatal. When the 29,751-nucleotide RNA genome of the SARS virus was sequenced, it proved to be a completely new form of coronavirus, not closely related to any of the three previously described forms. Virologists in 2005 identified the Chinese horseshoe bat as the natural host of the SARS virus. Because these bats are healthy carriers not sickened by the virus, and occur commonly throughout Asia, it will be difficult to prevent future outbreaks.



West Nile Virus. A mosquito-borne virus, West Nile virus first infected people in North America in 1999. Carried by infected crows and other birds, the virus proceeded to spread across the country, with 4,156 cases at its peak in 2002, 284 of whom died. By 2005 the wave of infection had greatly lessened. The virus is thought to be transmitted to humans by mosquitoes that have previously bitten infected birds. Earlier spread of the virus through Europe also abated after several years.


Key Learning Outcome 16.10. Viruses are responsible for some of the most lethal diseases of humans. Some of the most serious examples are viruses that have transferred to humans from some other host.


Inquiry & Analysis

Does HIV Infect All White Blood Cells?

Humans are protected from microbial infections by their immune system, a collection of cells that circulate in the blood. Loosely called "white blood cells,” this collection actually contains a variety of different cell types. Some of them possess CD4 cell surface identification markers (think of them as ID tags). Cells that trigger antibody production when they detect virus-infected cells and macrophage cells that initially attack invading bacteria both carry CD4 ID tags. Other cells possess CD8 ID tags, such as killer cells, which are immune cells that bore holes into virus-infected cells. In an AIDS patient, neither CD4 nor CD8 cells actively defend against HIV infection. Are either or both of these cell types killed by the HIV virus?

To investigate this issue, researchers mixed together CD4-tagged cells (called CD4+ cells) and CD8-tagged cells (called CD8+ cells), and then added HIV to the mixture. HIV, colored red in the electron micrograph shown here, was then able to infect either kind of cell. The white blood cell culture was monitored at 5-day intervals for 25 days, taking a sample at each interval and scoring it for how many CD4+ cells and how many CD8+ cells it contained. The surviving percentage of each cell type in each sample is presented in the graph on the right above.




1. Applying Concepts

a. Variable. In the graph, what is the dependent variable?

b. Percentage. If the percentage of surviving cells decreases, what does this say about the absolute number of cells? Can the absolute number of cells increase if the surviving percent decreases? Explain.

2. Interpreting Data

a. Does the percentage of surviving cells change over the course of three weeks for CD4+ cells? For CD8+ cells?

b. Over the course of the three weeks, is there any obvious difference in the percentage survival of the two cell types? Describe it. How would you quantify this difference? [Hint: Plot the ratio of surviving CD4+ to surviving CD8+ cells versus days after infection.]

3. Making Inferences. What would you say is responsible for the difference in percentage of surviving cells between the two cell types? How might you test this inference?

4. Drawing Conclusions

a. Is either type of white blood cell totally eliminated by HIV infection over the course of this experiment?

b. Is either type of cell virtually eliminated? If so, which one?

c. Is either type of cell not strongly affected by HIV infection? If so, which one? Can you think of a reason why the percent surviving cells of this cell type changes at all? How might you test this hypothesis?

5. Further Analysis Neither CD4+ nor CD8+ cells actively defend AIDS patients. If one of them is not eliminated by HIV, why do you suppose it ceases to defend HIV-infected AIDS patients? Can you think of a way to investigate this possibility?



Test Your Understanding

1. Which is not an assumption used in the Miller and Urey experiment?

a. The primordial atmosphere of the earth contained methane.

b. The primordial atmosphere of the earth contained the same amount of oxygen as today.

c. Lightning provided energy for chemical reactions.

d. Small inorganic chemicals used to make larger organic molecules were present in the atmosphere and the water.

2. While it is still unknown how the first cells formed, scientists suspect that the first active biological macromolecule was

a. protein.

b. DNA.

c. RNA.

d. carbohydrates.

3. Bacteria

a. are prokaryotic.

b. have been on the earth for at least 2.5 billion years.

c. are the most abundant life-form on earth.

d. All of the above.

4. Some species of prokaryotes are able to obtain carbon from CO2 and energy by oxidizing inorganic chemicals. These species are also called

a. photoautotrophs.

b. chemoautotrophs.

c. photoheterotrophs.

d. chemoheterotrophs.

5. Which of the following can be attributed to bacteria?

a. decomposition of dead organic matter

b. increasing oxygen levels in the atmosphere

c. insect resistance in plants

d. All of the above.

6. Cyanobacteria are thought to have been very prominent in earth’s history by

a. making nucleic acids.

b. making proteins.

c. producing the carbon dioxide that is in the atmosphere.

d. producing the oxygen that is in the atmosphere.

7. Viruses are

a. protein coats that contain DNA or RNA.

b. simple eukaryotic cells.

c. simple prokaryotic cells.

d. alive.

8. A viral reproductive cycle in which the virus enters the cell, uses the cell structures to make more viruses, then breaks open the cell to release the new viruses is called the

a. lysogenic cycle.

b. lambda cycle.

c. lytic cycle.

d. prophage cycle.

9. Animal viruses enter animal cells by

a. exocytosis.

b. matching a marker on the surface of the virus to a complementary marker on the surface of a cell.

c. contacting the host cell with protein tail fibers.

d. contacting any place on the cell’s membrane with the virus’s protein coat.

10. HIV is a virus that contains RNA. To insert its genetic material into the cell’s genome, HIV must

a. use the cell’s ribosomes and enzymes to read the RNA backward to produce DNA.

b. do nothing; the virus RNA will insert itself into the cell DNA without changing.

c. use an enzyme called reverse transcriptase to convert the virus RNA to DNA, which is then integrated into the cell’s chromosome.

d. No answer is correct.