Boiling is often used as a readily available way to sterilise things
and water companies rely upon "boil water" notices to alert consumers
when a problem develops with the local water supply and the normal
water supply sterilisation system cannot be relied upon to be
sufficient.

How long should you boil water to ensure that all bugs have been
killed?

Boiling water continuously for 1 minute will inactivate most water-
borne pathogens, including encysted protozoa such as Cryptosporidium -
a common contaminant of water supplies.

However, bacterial spores, such as the spores of Bacillus anthracis,
the bacterium that causes anthrax, are microorganisms that are very
resistant to inactivation by the boiling water. Unfortunately, not
only can anthrax bacteria linger for many years in infected soil,
they are also agents that have been used with devastating effect in
bioterrorist attacks.
[For more about anthrax and bioterrorism see:
http://www.pighealth.com/diseases/anthrax.htm ]

New research from the USA Environmental Protection Agency and the
U.S. Centers for Disease Control in Atlanta, Georgia has recently
investigated the resistance of spores of B. anthracis and three other
strains of Bacillus bacteria to destruction by boiling water.

Spores of all strains of the Bacillus bacteria analyzed in this study
were inactivated after boiling for 3-5 min in a covered vessel.

Spores still survived after 5 min of boiling in an open vessel.
Temperatures immediately above the surface of the boiling water in
the covered vessels averaged 98.9°C, while the temperature
immediately above the water level in the uncovered vessels averaged
77.3°C.

According to a literature review citing published reports dating back
to 1882, boiling times reported to destroy B. anthracis spores have
previously varied over a range of 1 to 12 min.

NOTE: Boiling time refers to the total time the water is held at a
rolling boil and should not be confused with the first sign of
bubbles from dissolved gases in the water. Since water boils at lower
temperatures at higher altitudes (approximately 90°C at 3 km),
boiling times must also compensate for decreased atmospheric pressure
conditions.

Reference
---------
"Boiling and Bacillus spores" by Eugene W. Rice, Laura J. Rose,
Clifford H. Johnson, Laura A. Boczek, Matthew J. Arduino and Donald
J. Reasoner. Emerging Infectious Diseases, Volume 10 Number 10
October 2004.