However, a small number, around 100 known species, are able to cause disease.
It is known to cause disease in both animals and man.
The bad air could also come from within the earth, and thereby cause the disease.
In some cases doctors don't know what caused the disease.
Under normal circumstances they do not cause disease, but may even help our health.
At this time nobody in the world knew what caused disease.
Some say it is the water that causes the disease.
Doctors still do not know what causes the disease, or why.
Could something at the television studio have caused the disease in all four people?
Of the 300 or so viruses discovered in the last 15 years, about 10 are known to cause human disease.
