Charles Manson is shown in this 2011 picture from the California Department of Corrections and Rehabilitation. (Handout/Reuters)

David Smith founded the Haight Ashbury Free Clinic, which is now part of HealthRIGHT 360 in northern California.

Fifty years ago, at the height of the "summer of love," up to 30,000 hippies crowded San Francisco's Golden Gate Park, dancing to the psychedelic music pioneered by bands such as the Grateful Dead. The event attracted thousands of young people to the Haight-Ashbury district in San Francisco — a small neighborhood that soon became the epicenter of a counterculture movement centered on the philosophy that chemistry and drugs could improve life experiences.

This was the complex environment that wrought Charles Manson, the notorious cult leader of the murderous Manson Family who would become a national obsession. I studied Manson, who died Sunday at age 83, while researching psychoactive drugs in the late '60s, watching as he became an infamous symbol of the dark side in the summer of love — and helped to stunt progressive policies for decades to come.

I first interacted with Manson and the women who followed him as a physician in 1967 while treating them at the health clinic I launched to serve the growing population of young drug users in Haight-Ashbury. I quickly discovered that their philosophy smacked of delusions of grandeur, fueled by LSD and mind control. Manson used his influence over the young female members of the Manson Family to control their behavior, changing their names to dissolve their identities. The philosophy was bizarre — but so was the whole of Haight-Ashbury, and Manson just seemed another character moving through our scene.

My next interaction with the group was in 1969. By this time, hippies had begun moving out of Haight-Ashbury and into the country, so we formed health groups to provide visiting nurses to their communes. We saw this as an opportunity to study the new counterculture communal phenomenon, prompting my clinic's administrator, Al Rose, to temporarily live with and study the Manson Family on their ranch.

Rose brought back interesting field notes that served as the basis of our paper "The Group Marriage Commune: A Case Study," published in the Journal of Psychedelic Drugs and completed before Manson Family members began killing people. We found that Manson served as the group's absolute ruler, often performing drug-involved magic tricks to show his mental powers. The group consisted of 20 core members. Manson would use the sex lives of the women to determine who was truly committed to the philosophy and his control.

Failure to adapt brought discipline; many were asked to leave involuntarily. Failure to conform to Manson's philosophy or refusal to have sex with any member of the group were also grounds for expulsion. The Family's members never discussed murder or violence with us, but in an interview with member Susan Atkins in 1980 for her parole hearing, it became clear that Manson had probably formulated his "Helter Skelter" worldview — which predicted a race war in the United States — while they were using methamphetamines, leading to the infamous Tate/LaBianca murders in 1969.

Atkins said she spent five years in prison before Manson "left her brain." We know now that drug-infused mind control is a very real phenomenon among such susceptible youths. Further, it is difficult to identify and anticipate. Society still has not fully grasped the danger when drugs, cult leaders and alienated youths mix. With the growing drug problem today, there is a potential for more violent cult acts to occur.

For many, the Manson episode validated their fears of the counterculture movement. But while there were legitimate negatives to the era, there was also real good happening — such as new forms of psychedelic music; increased racial, gender and religious tolerance, as well as acceptance of gays and lesbians; and the advance of the theory that addiction should be treated as a disease in mainstream medical settings.

I observed these changes firsthand. In May 1967, the San Francisco Health Department had refused our request to set up a free health clinic in the area. Health officials resisted the idea of serving long-haired, drug-taking, sexually promiscuous hippies, hoping instead that such people would just go away. This was the atmosphere that gave birth to the phrase "health care is a right, not a privilege" — which eventually became the protest statement for the national free-clinic movement. Half a century later, the slogan undergirded President Barack Obama's Affordable Care Act and the fight now to preserve it during the Trump presidency. Few people understand how it evolved from the counterculture movement.

Charles Manson and his Family took advantage of this progressive movement to advance their own ideas of death, destruction and a tarnishment of moral principles. It is only despite the horrors of Manson's crimes that the spirit of the "summer of love" survives today.