Allergies are caused by the body's overzealous reaction to normally harmless substances.

The normally harmless brook can become a danger of high waters quickly and has already been a threat several times.

Compsognathus: Creatures that are normally harmless, but will sometimes attack if frightened.

These bacteria are normally harmless but are triggered off by a rise in temperature.

However, it suppressed the immune system's ability to control normally harmless viruses, which led to fatal brain infections.

Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment.

Styes are normally harmless and do not cause long lasting damage.

E. coli, a common bacteria that lives in the intestines of humans and animals, is normally harmless.

The debris from the event and other normally harmless materials will be contaminated with radiation.

E. coli is a common gut bacteria that is normally harmless.