The military experts then say that the virus has undergone "extreme amplification".

The virus originated 50-150 years ago and has undergone a recent population expansion.

Vaccination has proved difficult because there are so many viruses involved and the viruses undergo rapid change.

A virus itself undergoes no change of matter and energy; it does not eat nor does it excrete.

One possibility is that the virus has undergone changes that eased its transmission to humans.

However, there have been a few cases in which this virus underwent human to human transmission.

Once within a muscle or nerve cell, the virus undergoes the replication.

They found that the viruses had undergone a subtle mutation, or change in their genetic structure.

The viruses are not new but have undergone an exchange of genes.

Previously it has been thought that the virus does not undergo cross species infection.