WORM'S EYE

Viruses exist to nab your cells and use them for their own reproductive purposes. They have to, because a virus is nothing more than a few strands of rogue DNA (or rogue RNA, DNA's single-stranded cousin) wrapped in a protein coat to keep the draft out.

They are not cells, and they have none of the internal structures that cells use to go about the business of life, which is, generally, to make more life. No, viruses are just genetic material looking for a free ride--looking to hijack a host cell and make its machinery do the invader's reproductive bidding.

Rule for Viral Success #1:
Mutation, Mutation, Mutation

With so little to call their own, how have these biological pirates survived for so long? The answer lies in two traits that give viruses superb evolutionary advantages: superfast reproduction and genetic mutations.

Viruses live to reproduce. Although they must do this within host cells, once inside, viruses replicate with enough abandon to shame a rabbit. Like some evil biological Xerox machine, they reprogram the machinery that host cells normally use to copy their own DNA and use it to spit out copy after copy of themselves.

Genetic mutations add insult to injury. With so much reproduction going on, viruses can mutate almost as fast as they propagate. And each mutation stands a good chance of conferring advantageous survival and adaptation characteristics on the next generation of viral invaders.

Rule for Viral Success #2:
Pick a Likely Victim

Viruses invade all kinds of cells, including plant cells, animal cells, fungi, even bacteria. Each virus tends to have a very specific M.O., though. Which host cells look like the best victims to any given virus depends on the unique proteins found on a virus's protein coat, as well as on the protein receptors found on the poor target cell.

Some viruses recognize the general receptors that occur on many different kinds of cells. The virus for rabies, for example, can invade so many different kinds of cells that it can span species, infecting rodents, dogs, and humans.

Other viruses are more restricted and can invade only specific kinds of cells. The common cold virus, for example, can invade only the cells lining the human upper respiratory tract. It's a picky thief.

Rule for Viral Success #3:
Make It an Inside Job

Viral entry mechanisms are as diverse as viruses themselves, which is why viruses often elude drug treatments. Some enter a target cell by binding to a specific receptor and passing through the host cell membrane to the cell interior. Others don't even need to enter the cell, but simply attach to the surface and use a needle-like structure to inject their DNA right in.

Once viral genes are inside, the virus begins its cycle of replication. It exploits the host cell's supplies and machinery, forcing it to copy viral genes and to synthesize more viral protein coats. Then, these two components come together to form copies of the virus that emerge from the host cell.

Sometimes they "bud" off the cell, like bubbling on top a simmering stew. At other, more violent times, copies simply fill the cell until it can hold no more; it explodes, releasing its viral hoard into the surrounding area.

Either way, the viral progeny go on to infect new cells--and the cycle starts again. Disease symptoms can and do result from this cellular damage. Most often, though, the sickness you feel is the result of your immune system's response to the foreign invader. And make no mistake, it will respond.

Rule for Viral Success #4:
Avoid the Cops

Your immune system's first-responders act like beat cops on patrol 24/7. Should they see anything amiss while walking the body's beat, they will make arrests. One kind of cellular cop, the phagocytes, will engulf strange viruses and digest them. Another kind, natural killer cells, recognizes suspect changes on the surface of infected cells and will release chemicals to disintegrate both virus and cell alike.

After spotting the infection, your body can launch a more specific and intensive attack. Proteins called antibodies surround, bind to, and neutralize viruses and other invaders in your bloodstream. Killer T-cells mercilessly destroy infected cells and halt systemic infection. Both help your body remember the infection and mount a faster response to the same invader next time.

Still other players merit mention. When a cell does get infected with a virus, sometimes it manages to secrete small proteins called interferons that serve to warn neighboring cells of an imminent viral invasion. These "Paul Revere" proteins work by encouraging neighboring cells to synthesize proteins that can interfere with viral replication.

Christina Catron and Michael Himick
October 27, 2004

Want to learn more?
Read what the CDC says about the flu virus
http://www.cdc.gov/flu/about/disease.htm

Get answers from the CDC about the flu vaccine
http://www.cdc.gov/flu/about/qa/0405season.htm

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