Nineteen cases of a potentially fatal form of measles have been reported in Santa Rosa, Hayward and Fremont since the first of the year, health officials said. The number of cases could grow to 100 or more in the next three months, said Dr. Loring Dales, a medical epidemiologist with the Bureau of Infectious Disease Control at the State Department of Health Services in Berkeley. Although it is usually considered a childhood ailment, the "10-day measles" can strike people of all ages and is most dangerous in adults. People who have had the disease once or are vaccinated are immune. None of the 19 cases developed complications. But in about one case in 1,000, the virus causes pneumonia or invades the brain, causing encephalitis. The death rate is about one in 3,000 infected children and about one in 800 infected adults, Dales said.