About 8:15 p.m. on the evening of February 19, 1994, Ramirez, suffering from severe heart palpitations, was brought into the emergency department of Riverside General Hospital by paramedics. She was extremely confused and was suffering from tachycardia and Cheyne–Stokes respiration.

The medical staff injected her with diazepam, midazolam, and lorazepam to sedate her. When it became clear that Ramirez was responding poorly to treatment, the staff tried to defibrillate her heart; at that point several people saw an oily sheen covering Ramirez's body, and some noticed a fruity, garlic-like odor that they thought was coming from her mouth. A registered nurse named Susan Kane attempted to draw blood from Ramirez's arm and noticed an ammonia-like smell coming from the tube.

She passed the syringe to Julie Gorchynski, a medical resident, who noticed manila-colored particles floating in the blood. At this point, Kane fainted and was removed from the room. Shortly thereafter, Gorchynski began to feel nauseated. Complaining that she was lightheaded, she left the trauma room and sat at a nurse's desk. A staff member asked her if she was okay, but before she could respond she also fainted. Maureen Welch, a respiratory therapist who was assisting in the trauma room, was the third to pass out. The staff was then ordered to evacuate all emergency department patients to the parking lot outside the hospital. Overall, 23 people became ill and five were hospitalized. A skeleton crew stayed behind to stabilize Ramirez. At 8:50 p.m., after 45 minutes of CPR and defibrillation, Ramirez was pronounced dead from kidney failure related to her cancer

The county health department called in California's Department of Health and Human Services, which put two scientists, Drs. Ana Maria Osorio and Kirsten Waller, on the case. They interviewed 34 hospital staff who had been working in the emergency department on February 19. Using a standardized questionnaire, Osorio and Waller found that the people who had developed severe symptoms, such as loss of consciousness, shortness of breath, and muscle spasms, tended to have certain things in common. People who had worked within two feet of Ramirez and had handled her intravenous lines had been at high risk. But other factors that correlated with severe symptoms did not appear to match a scenario in which fumes had been released: the survey found that those afflicted tended to be women rather than men, and they all had normal blood tests after the exposure. They believed the hospital workers suffered from mass hysteria.

Gorchynski denied that she had been affected by mass hysteria and pointed to her own medical history as evidence. After the exposure, she spent two weeks in the intensive care unit with breathing problems. She developed hepatitis and avascular necrosis in her knees.