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Viruses and Human Disease

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Viral or bacterial?

Doesn't look like fun. The flu is caused by an influenza virus. And usually a slightly different virus every season.

Viruses and Human Disease

Viruses cause many human diseases. In addition to the flu and HIV, viruses cause rabies, measles, diarrheal diseases, hepatitis, polio, cold sores and other diseases (see Figure below ). Viral diseases range from mild to fatal. One way viruses cause disease is by causing host cells to burst open and die. Viruses may also cause disease without killing host cells. They may cause illness by disrupting homeostasis in host cells.

Cold Sore. Cold sores are caused by a herpes virus.

Some viruses live in a dormant state inside the body. This is called latency . For example, the virus that causes chicken pox may infect a young child and cause the short-term disease chicken pox. Then the virus may remain latent in nerve cells within the body for decades. The virus may re-emerge later in life as the disease called shingles. In shingles, the virus causes painful skin rashes with blisters (see Figure below ).

Shingles. Shingles is a disease caused by the same virus that causes chicken pox.

Some viruses can cause cancer. For example, human papillomavirus (HPV) causes cancer of the cervix in females. Hepatitis B virus causes cancer of the liver. A viral cancer is likely to develop only after a person has been infected with a virus for many years.

The Flu

Influenza, or flu, is a contagious respiratory illness caused by influenza viruses. Influenza spreads around the world in seasonal epidemics. An epidemic is an outbreak of a disease within a population of people during a specific time. Every year in the United States, about 200,000 people are hospitalized and 36,000 people die from the flu. Flu pandemics can kill millions of people. A pandemic is an epidemic that spreads through human populations across a large region (for example a continent), or even worldwide. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus. Most influenza strains can be inactivated easily by disinfectants and detergents.

Emerging Viral Diseases

Modern modes of transportation allow more people and products to travel around the world at a faster pace. They also open the airways to the transcontinental movement of infectious disease vectors. One example of this occurring is West Nile Virus , which scientists believe was introduced to the United States by an infected air traveler. With the use of air travel, people are able to go to foreign lands, contract a disease and not have any symptoms of illness until they get home, possibly exposing others to the disease along the way. See Virus Crisis at http://www.neok12.com/php/watch.php?v=zX0b0954067a71795a6c647b&t=Microorganisms for additional information.

Often, new diseases result from the spread of an existing disease from animals to humans. A disease that can be spread from animals to humans is called a zoonosis . When a disease breaks out, scientists called epidemiologists investigate the outbreak, looking for its cause. Epidemiologists are like detectives trying to solve a crime. The information epidemiologists learn is important to understand the pathogen, and help prevent future outbreaks of disease.

A deadly strain of avian flu virus named H5N1 has posed the greatest risk for a new influenza pandemic since it first killed humans in Asia in the 1990s. The virus is passed from infected birds to humans. Fortunately, the virus has not mutated to a form that spreads easily between people.

Several lethal viruses that cause viral hemorrhagic fever have been discovered, two of which are shown below. Ebola outbreaks have been limited mainly to remote areas of the world. However, they have gained extensive media attention because of the high mortality rate—23 percent to 90 percent—depending on the strain. The primary hosts of the viruses are thought to be apes in west central Africa, but the virus has also been isolated from bats in the same region.

The Ebola virus (left), and Marburg virus (right), are viruses that cause hemorrhagic fevers that can cause multiple organ failure and death.

People get exposed to new and rare zoonoses when they move into new areas and encounter wild animals. For example, severe acute respiratory syndrome ( SARS ) is a respiratory disease which is caused by the SARS coronavirus. An outbreak in China in 2003 was linked to the handling and consumption of wild civet cats sold as food in a market. In 2005, two studies identified a number of SARS-like coronaviruses in Chinese bats. It is likely that the virus spread from bats to civets, and then to humans.

Summary

  • Viruses cause many human diseases by killing host cells or disturbing their homeostasis.
  • Viruses are not affected by antibiotics. Several viral diseases can be treated with antiviral drugs or prevented with vaccines.

Practice I

Use these resources to answer the questions that follow.

  1. Describe the influenza virus.
  2. Describe HIV.
  1. How does a virus infect a host cell?
  2. List 3 human illnesses caused by viruses.
  3. Why are epidemics and pandemics of concern?

Practice II

Review

1. How do viruses cause human disease?

2. What is an epidemic? Why can the flu be considered an epidemic?

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