Expert: Florence M Rollwagen - 8/21/2008

Question
QUESTION: Hallo,
I would like to know how long does a human cell(no matter which one) live outside of the body?
And to which cells exactly are viruses attached?

Thank you very much in advance

ANSWER: Hi Diana:  Thanks for your question.

This is a very broad subject, and we'll need to specify somewhat exactly what we're talking about.

Many human cells can live for long periods outside the body in special culture conditions called tissue culture.  Some cells have been cultured for 20 years or more under these conditions.

If you mean how long does a cell live outside the body without laboratory help, not very long.  Certainly a matter of hours, not more than a day.  The usually die from dehydration.  

Viruses attach to specific cells via their receptors.  The receptors fit only certain cells.  For example, the cold virus (rhinovirus) has receptors only for respiratory cells.  Hepatitis virus for liver cells, etc.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD

---------- FOLLOW-UP ----------

QUESTION: Hallo,
Thank you very much for the answer.
It helped me to understand that no viruses can exist outside of the human body without laboratory help longer than 24h.Did i understand correct?
You say cells die from dehydration.Don't they die sooner from lack of oxygen (rather than Dehydration)?.I read somewhere it is a matter of seconds they die from lack of oxygen.Is that true?
I understand,if a cell dies,the virus dies together with the cell.Is that correct?

Thank you once again for the wonderful answers.

Diana

ANSWER: Hello again, Diana:  The 24 hour rule is not cast in stone, I'm just saying that cells die very quickly outside the body if they are not given protection.  As for the lack of oxygen, cells in the laboratory use oxygen from the air, so it's not really a problem.

Let's say you cut yourself with a knife in the kitchen. A drop of blood lands on the sink.  The blood cells are capable of metabolism using oxygen in the air and nutrients in the blood for many minutes or hours.  But the drop of blood will dry out (dehydrate) after a while, so the cells will certainly die then.

As for the viruses, nobody uses the word "die" when talking about them.  We sort of consider viruses to be in a gray area between the black and white of life and death.  Viruses can live for a long time in the dehydrated state.  But since they are only "alive" when inside the cell, we can't really say that they are dead.  Ultraviolet light (sunshine) can kill viruses, as can high heat because they denature the virus nucleic acids (DNA or RNA).  

If a virus infected cell is killed by the immune system, the virus dies because it has not been assembled yet, it is only in pieces inside the cell.  But this is different from "killing" an intact virus.

Hope this helps.

FM Rollwagen, PhD

---------- FOLLOW-UP ----------

QUESTION: Hallo,
Thank you very much for the quick respond.It was very helpful.
I still cannot understand something.
If a virus lives by a cell's metabolism,how can a virus stay alive if the cell is totally dead?
"Viruses can live for a long time in the dehydrated state"
How is this possible?
Where could i read about isolated viruses and their structure ( for example Hepatitis B)?

Thank you very much in advance
Diana

Answer
Hi Diana:  As I said above, viruses are not really "alive" or "dead".  They are a gray area between those two extremes.  When the virus is outside the cell, it is "dead" in the sense that it is not reproducing.  When the virus enters the cell, it becomes "alive" because it uses the cell's metabolic sytems to reproduce.  So even though a virus can "live" for long periods ouside the cell, they only become "alive" after they are incorporated into a cell.

I guess I should have said Viruses can be infectious for a long time, rather than "alive" for a long time.

This would have avoided some confusion.

If you want to look up information about infectious agents I recommend three websites:

www.cdc.gov  This is the Centers for Disease Control in Atlanta. Click on "Diseases and Conditions" They have great information for both the public and for health professionals.  So you can get as much information as you want.  

Another site is the National Institute of Allergy and Infectious Diseases in Bethesda, MD.
http://www3.niaid.nih.gov/  Just click on "Health and Science Topics"

The third site is the National Library of Medicine http://www.nlm.nih.gov/  Click on "Health Topics"

I hope these answers help you.  Please write back if you have more questions.

FM Rollwagen, PhD