Healthy people over 65 years of age seem to have some natural immunity.

The problem with natural immunity is the risk of complications.

Virginia called for volunteers from the lower South where the disease was more common and residents had developed some natural immunity.

A cleaner environment left increasing numbers of children with no natural immunity.

In fact being a carrier helps to boost natural immunity.

But most of us have a natural or easily acquired immunity.

An epidemic broke out among the Cherokee, who had no natural immunity.

"I have a natural immunity," she said and looked at her hands.

It is very hard to catch - 95 percent of humans have a natural immunity.

Because it's a new virus, most of us don't have any natural immunity to it.