Sue,

The effectiveness of boiling the water to kill microorganisms is influenced by the combination of temperature and length of treatment, and depends on the species of microorganism.

However, because of practical reasons (most of us don't have the tabulated values for the species-temperature-time combinations, or thermometers, or stop watches at hand when we're boiling water at home), the attainment of the rolling boiling stage is usually considered as the end point of the process.

Armando