Whooping cough, a disease for which Americans have had a vaccine available since the 1920s, is on pace to infect a record number of Contra Costa County residents in 2011. Last year was bad enough, with a record 205 cases reported — but this year is looking even worse, Contra Costa Health Services reported yesterday. To date, at least 78 cases have been recorded in Contra Costa out of 1,080 statewide — more than 7 percent of the total. Contra Costa County’s population of 1.03 million, by contrast, is only 2.8 percent of California’s 36.96 million. That’s an infection rate of 7.5 per 10,000 in Contra Costa versus 3 per 10,000 in the state as a whole.