However, researchers have not identified just what those causes might be.

VOA: special.2009.01.06

Scientists are still not sure what causes Alzheimer's disease.

VOA: special.2009.09.15

Doctors do not know what causes this process.

VOA: special.2009.01.06

Scientists are not sure what causes tension headaches.

VOA: special.2010.03.30

Doctors do not know what causes asthma.

VOA: special.2009.07.28

Experts are not sure what causes lupus.

VOA: special.2009.04.14