"There's a lot of virus circulating out there," he said.

"The virus is circulating now between wild birds and mosquitoes," he said.

The amount of virus circulating in the region will likely increase as the weather cools.

This suggests the virus had been circulating for years or perhaps decades in the Kinshasa population.

By the end of the trial, 80 percent had the amount of virus circulating in their blood driven to very low levels.

The viral load test only detects the amount of virus circulating in the blood (about 2%).

In the maintenance stage the virus remains on the sheep's lymphocytes and circulates the body.

The virus now circulating in birds does not attach easily to cells inside human noses or throats.

Exploring the influence of a mass gathering on community influenza activity is complex, especially when numerous viruses circulate during such an event.

There is no program to monitor what viruses are circulating in pigs in the United States, however.