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The tally of Pima County residents sickened by the West Nile virus has climbed to nine this season, signaling a very mild outbreak so far.

However, six of those victims have suffered the most severe form of West Nile illness, with the virus causing brain-affecting meningitis or encephalitis.

And another five people here are known to be infected but have shown no symptoms to date. Their infections were detected when they tried to donate blood.

"We are following these donors to see if they go on to develop illness. So far, so good," said Lisa Hulette, epidemiologist at the Pima County Health Department.

If they don't become ill, that won't be unusual. Most people infected with West Nile — fully 80 percent — never know it because they suffer few or no symptoms.

But the rest will develop a flu-like illness known as "West Nile fever." And about 1 percent will progress to neuro-invasive disease — starting with high fever, severe headache and stiff neck — that can result in permanent disability or death.

For that reason, confirmed cases of West Nile signal only a tiny percentage of overall infection in the population, since the vast majority of cases go undetected and unreported.

Unlike Maricopa County, where the year's first West Nile death was reported this week, Pima County has never endured a severe outbreak. Last year's case count — at 47, with three deaths — was by far the area's worst since the virus entered the state in 2003.

However, that didn't come close to the West Nile epidemic that struck Maricopa County in 2004, when the virus sickened 357 people, killing 14.

By this time last year, 28 cases had been confirmed in Pima County. The outbreak continued well into November.

"Yes, this looks to be a milder year, but we're not done yet," Hulette said. "There are still mosquitoes out there, so people must still take precautions. We fully expect more cases during the next four to six weeks, until the season is over."

Mosquito activity typically drops after two or three freezing nights. Nighttime lows are hovering around 60 degrees, but may fall into the 50s through the weekend.

Humans and horses are infected with West Nile through the bite of an infected mosquito, which has fed on a sick bird. The virus cannot be transmitted from human to human, or from horse to human.

Also unlike last year, when many of the cases were clustered in a Midtown "hot zone," this year's cases are spread throughout the county, Hulette said. The victims are all adults, ranging in age from 40 to 83.

The virus also has been confirmed in two horses, two wild birds, one test chicken and three mosquito pools.

Four more suspected human cases are under investigation.