The Sacramento Bee reports Tuesday that its analysis using ZIP codes and U.S. census demographics showed that entrenched poverty was the key commonality in the hard-hit areas.
The influenza season has been particularly bad in south Sacramento, with at least 22 intensive care unit cases and 11 flu fatalities.
The number of flu-related deaths has increased again in California -- by 35 in the past week -- bringing the total number of deaths to 278 this season.
California is reporting almost twice the number of flu-related deaths as last year.
The County Department of Health and Human Services says the most recent victim was a 53-year-old man who died at home.
California health officials are reporting the rate of illness from influenza is higher than normal for this time of year, with a more dangerous form of the virus leading the way.