18.3, viral diseases

• Virus


  A submicroscopic infectious agent that replicates only inside the living cells of an organism
• Virus


  • Your body has defenses against viruses
  • Skin is your body's first defense
  • A virus cannot get through your skin unless there is an opening such as a cut or a scrape
  • A virus can also enter your body through other openings, such as the mouth, nose, genital areas, eyes, and ears
  • Some viral infections are deadly and others are barely noticeable
• Common cold


  • More than 200 viruses cause the illness
  • It is hard to find a cure because it is caused by so many different viruses that mutate rapidly
• Influenza


  • The influenza virus is often called the flu for short
  • The flu spreads quickly
  • The flu can be controlled with a vaccine
  • Flu viruses mutate rapidly, so a new vaccine must be made every year
• SARS


  • SARS (severe acute respiratory disease) has symptoms similar to the flu, including fever and coughing
  • SARS first appeared in Asia in 2002
  • By the following summer it had spread to other countries
• HIV


  • HIV (human immunodeficiency virus) is a type of virus called a retrovirus
  • Retro- means "backward"
  • A retrovirus copies its genetic material "backwards"
  • Usually, DNA is used to make a copy of RNA, but a retrovirus contains RNA that gets used to make DNA
  • The DNA then becomes part of the host cell's DNA
  • HIV can remain part of the host cell's DNA for many years with no symptoms
  • When the virus becomes active, it makes more viral particles
  • The host cells burst, and the new viruses can infect other cells
  • This stage of the disease destroys the host's white blood cells
  • The loss of white blood cells leads to AIDS, or acquired immune deficiency syndrome
  • HIV mutates very quickly, which makes it a challenge to treat
• Vaccine


  • A vaccine contains the weakened virus or parts of the virus that it is supposed to protect against
  • A vaccine triggers the body's immune system response
  • Because the vaccine contains a weakened virus, it does not cause illness
  • But if the virus enters the body again, the body will be prepared to start an immune response before the virus can cause damage
• Every year, millions of people are vaccinated against influenza