This year's outbreak of meningococcal disease in Los Angeles County is the worst in the last 30 years, according to the county Department of Health Services. But the number of new cases declined over the summer months, as expected. Through the end of August, 227 cases of the severe bacterial infection, including 38 deaths, had been reported, a department spokeswoman said. By comparison, in the first eight months of 1986, there were only 84 cases and 17 deaths. The previous record number of cases was in 1966, when 199 cases were diagnosed. The meningococcal infection causes spinal meningitis and blood poisoning. One possible explanation for the outbreak is a shift in the strain of bacteria, which has been detected in the county over the last several years. As a result, more people may be susceptible to the new strain.