Between January 1 and August 15, 1985, 142 cases of listeriosis were identified in Los Angeles and Orange Counties, California. Ninety three cases (65.5%) occurred in pregnant women or their offspring and 49 (34.5%) in nonpregnant adults. There were 28 deaths-10 neonatal deaths and 18 nonpregnant adults. There were an additional 20 fetal deaths. Of the nonpregnant adults, 98% (48 of 49) had a known predisposing condition.

A case control study implicated Mexican-style soft cheese (OR 5.5; 95% CI 1.2 to 24.8) as the vehicle of infection. A second case-control study showed an association with one brand, a cheese manufactured by Jalisco Products, Inc., (OR 8.5, 95% CI 2.4 to 26.2). Four packages of Jalisco cheese, queso fresco and cotija, products grew Listeria monocytogenes, serotype 4b. An investigation of the cheese plant suggested that the cheese was commonly contaminated with unpasteurized milk.