No one knows how the virus came to the United States.

Initially, researchers had no idea where the virus was coming from.

"The virus cannot have come through the net," he said.

He would remain convinced the virus had come through the net.

We never found out where the virus came from.

I'd say there's a good chance that's where the virus came from.

You said the viruses come from various areas in Africa.

The Enterprise and its fleet then set out to find where the virus came from.

What wasn't immediately clear was where the virus had come from.

Needless to say, we'd love to find out where this virus came from.