For every person who contracts the most severe form of West Nile virus — life-threatening brain swelling called meningitis and encephalitis — state health officials estimate there are 140 others infected with the disease.

Of those 140 cases:

• 20 percent will become ill with symptoms.

• 80 percent won't get sick or have any symptoms at all.

As of Wednesday afternoon, nine Coloradans had contracted a severe "neuro-invasive" West Nile case. Multiplying nine by 140, health officials estimate that roughly 1,260 people are infected statewide — although the majority have no symptoms.Source: Colorado Department Of Public Health And Environment

The psychology of the numbers

Health officials know that people's perception of risk changes with the numbers.

"It's human nature that people perceive risk as higher the closer it is to them — if there's a West Nile case in the next town, they will perceive the risk as lower than if it's on their block," state epidemiologist John Pape said.

Risk is also perceived as higher as the number of cases goes up. But people should know that's just the perceived risk. The relative risk may be higher.

"We don't want to scare people, but the message right now is the risk is high and you should be avoiding mosquito bites," Pape said. "The rest of the numbers are window dressing."