So why did transmission not occur until the late 1950's?

Other studies suggest transmission can also occur through a woman's father.

The next live transmission will not occur until 8 December 2000.

They said there was compelling evidence that transmission had occurred in the office.

Transmission between pigs and humans have occurred in the past.

Transmission of this virus through the air or blood does not normally occur.

Mechanical transmission also occurs once it has been introduced to an area.

Still other studies point out that cultural transmission can occur without actual human migration.

It is not yet known how much further transmission has occurred among humans, or whether the viruses can cause disease.

No one knows why tertiary transmission is not occurring in America.