Angelica Pathan, an epidemiologist at Mount Sinai Hospital in New York, started trying to investigate the cause of the spike in autism cases in the early 1990s. She and her colleagues identified at least four distinct etiologies—”nonsynonymous” genetic defects, rare chromosomal disorders, childhood vaccinations, and other environmental triggers—but they couldn’t determine which one was causing the increase in cases. Because the rates of autism have been increasing rapidly in recent years, she continued, “it was logical to do a statistical study.”

Pathan and her colleagues found that autism diagnoses peaked in the 1990s, as children received stronger vaccinations to protect against infectious disease. By the 2000s, the rate had dropped dramatically and remained relatively steady until the present day—the year Autism Diagnostic Observation Schedule began. In all, her team identified a total of 1,924 documented cases of autism from the early 1990s, compared to 7,835 from the 2008 national database. “It’s pretty devastating,” Pathan said. (A similar study in 2000 looked at 2,664 cases, and a recent update identified more than 11,000 cases.) Of those 8,650 in 2012, 3,527 (61 percent) are autistic. In a 2011 Pediatrics article, Pathan predicted that “the total number of [autistic] cases could rise to more than 20,000 by 2025.”

Pathan and her colleagues believe it is possible that an individual could have autism even if they have not had any measles, whooping cough, measles mumps, rubella, polio, polio vaccine, or pertussis in the past three months. (This can be a sign that the measles virus could be hiding in the body; see “Why Autism Might Be More Common Than Previously Thought.”) What’s more, these cases tend to coincide with outbreaks of other illnesses—perhaps the seasonal increase in pertussis might contribute to autism’s spike.

Despite the high rates, the incidence of these diseases (particularly measles) were low until the first part of the 20th century. Between 1900 and 1960, there were four measles deaths in the United States per million people—two measles deaths are the commonest childhood disease in the world; most of the time, about 99 percent of cases go undiagnosed. In the years between 1960 and the 1970s, however, measles cases rose steadily—in the early 1980s, there were nearly 1 million