Scientists say a virus or a high temperature can actually be good for you because they trigger body chemicals which can prolong cell life.
Viruses could benefit cell health
So a little physical stress, like a little red wine or chocolate, may be beneficial.
Researchers writing in the journal Molecular Biology of the Cell say bursts of stress "turns on" key genes which encode protective molecules.
But they added that prolonged stress was not good.
Scientists from Northwestern University in Illinois, US, studied C. elegans, a transparent roundworm whose biochemical environment is similar to that of human beings.
They focused on heat shock factor, the master gene that controls the expression of molecular chaperones, special protective chemicals that respond to stress in a cell.
Molecular chaperones "tidy up" damaged proteins in cells. Stress triggers this reaction, prolonging life by preventing or delaying cell damage.
The scientists found that when heat shock factor genes were over-expressed, the worms' lifespan increased. But if they were under-expressed, the opposite was true.
The researchers highlighted increased temperatures, oxygen stress, bacterial and viral infections, and exposure to toxins such as heavy metals as factors which could increase stress on cells.
Professor Richard Morimoto, who led the research, said: "Sustained stress definitely is not good for you, but it appears that an occasional burst of stress or low levels of stress can be very protective.
"Brief exposure to environmental and physiological stress has long-term benefits to the cell because it unleashes a great number of molecular chaperones that capture all kinds of damaged and misfolded proteins."
Phil Evans, professor of psychology at the University o Westminster, told BBC News Online: "Stress affects our minds and biological systems so wholly and profoundly
that there should be no problem in acknowledging its influence.
"Acute stress leads to all sorts of potentially measurable effects on our physical
selves, and the problem is working out which isolated effects may be significant in the
round and in the longer term for health."
"But there is, as the researchers acknowledge, lots of evidence that chronic and severe long-term 'stress' is bad for health."