SANTA ROSA, Calif. (AP)  -- A norovirus outbreak at four assisted and independent living centers in Sonoma County has sickened more than 100 over the past three weeks.

County health officials said 90 residents and 18 staff workers have fallen ill since June 20. They did not identify the facilities because of concern that doing so would discourage others from reporting outbreaks.

In 2006 and 2007, the norovirus, or severe stomach flu, sickened more than 1,500 people in 37 care facilities in Sonoma County. The illness was the contributing cause in the deaths of seven elderly people.

The Santa Rosa Press Democrat reports two of the facilities reporting outbreaks last week have been cleared, which means they haven't had new cases in 48 hours. Of the two others, one had another case over the weekend, and the other had five.