The County Department of Public Health says in January and February of 2014, 42 cases have been reported, compared to just two cases during the same period in 2013.
Pertussis is a highly contagious respiratory illness that is spread from person to person by droplets released when a person coughs or sneezes.
Infection can occur in any age group, although newborns and infants under 1 year of age are usually at highest risk.
To reduce the risk of infection, the health department recommends that pregnant women -- and anyone who lives or works with infants younger than 12 months of age -- should be vaccinated.
For additional information, visit the Fresno County Department of Public Health website at www.fcdph.org/pertussis or call Communicable Disease Investigations at 559-600-3332.