Bacteria and Viruses-How They Work

19 05 2011

There are thousands of types of bacteria and most are harmless or even beneficial.  However, even “good” bacteria, if they find their way to the wrong place, can cause harm.  For example, bacteria that live in our mouth can cause illness if they find their way to the middle ear and cause an ear infection.  Also, some bacteria that ordinarily do not cause disease in persons with a working immune system, may do so in people with a weakened immune defense system.

Most bacterial diseases occur when bacteria multiply rapidly in tissue, damaging or killing it.  Boils result from the multiplication of bacteria in the skin.  Other bacteria cause disease by producing toxins or poisons.  Tetanus is a disease that begins after bacteria that normally live in soil enter the body through a wound.  The bacteria produce a poison that affects muscles and nerves far away from the wound.

To cause illness in humans, bacteria need to be able to gain access to the human body, reach their unique place within the body and multiply there.  The human body has developed several strategies to make life as difficult as possible for disease-causing or pathogenic bacteria, but bacteria have also learned how to break down our defenses.

An infection by pathogenic bacteria can be seen as a miniature battle between bacteria and host.  Bacteria try to survive and feed and multiply, while the human body’s immune system tries to prevent this.  The resulting infection is a process with three possible outcomes:

  • The immune system wins and the bacteria are removed, possibly with the help of medications.
  • The bacteria win the ultimate battle and kill their host (bacterial infections are a major cause of death, especially for children and elderly people).
  • An equilibrium is reached in which host and bacteria live in relationship together and damage is minimized.

All viruses live to make more viruses, and they usually make more viruses by invading a host’s cell (for instance, one of the cells in our bodies) and using the host cell’s “machinery” to churn out more of themselves.

Once the viruses mature, they leave the host cell and go find many more host cells to set up shop in so that they can start churning out more of themselves.

Sometimes, there is a hitch in the churning process.  During viral replication, mutations can occur.

The mutation can be bad enough to interfere with the virus’s ability to duplicate itself.  Or, it might just create a new strain of the virus.  The influenza virus does this, which is why every year, each new strain of flu virus must be identified in order to make a vaccine that is effective against it.

Humans are able to fight off viruses in several ways:

  • Proteins called interferons help neighboring cells resist infection by the virus.
  • If interferons fail, the immune system kicks in and fights the infection by killing the virus floating around outside the host cells and killing infected host cells.  (HIV is the exception, because HIV infects cells of the immune system that are necessary to kill the infected cells.)
  • There are drugs that help the body fight certain viral infections.  They hinder or stop the replication of the virus and are known as antivirals/retrovirals.

This is one in a series of excerpts from PKIDs’ Infectious Disease Workshop.  We hope you find the materials useful – the instructor’s text and activities are all free downloads. 

Photo credit: measles – sanofi pasteur



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