CHICAGO (CBS) — In February, a Chicago-area resident with mild respiratory symptoms went to a funeral for a family friend. Within a few weeks, three people would be dead of COVID-19 and 13 others would be infected.

In a case study published by the Centers for Disease Control (CDC) Thursday, experts examined how one person with mild symptoms can set off an unstoppable outbreak of infections.

At the funeral, the first patient — dubbed “patient A1.1” in the study because the patient was the first in the cluster of cases — hugged family friends and shared a potluck meal.

Four of those people contracted COVID-19.

One of the patients — called “patient B2.1” in the study because they were the first case in the second family cluster — had to be hospitalized and later died of COVID-19. While that patient was receiving care, two family members who had also been to the funeral visited the hospital.

One of those patients — “patient B3.1” may have contracted the virus there, according to the study’s authors. That patient did not wear personal protective equipment (PPE) while visiting the hospital.

Meanwhile, the first patient, who was “still experiencing mild respiratory symptoms,” went to a birthday party at the home of a relative. That patient embraced all nine of the other party-goers, and shared food at the three-hour party, the study said.

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Within a week, seven of the nine people who went to the party developed COVID-19 symptoms, and two died.

The spread didn’t stop there. While one of the patients from the birthday party was sick with COVID-19, another family member, along with a home health care professional, cared for that patient without PPE. The family member also had contact with what the study’s authors described as a “household contact.” All three — the family member, the home health care professional and the household contact — developed COVID-19 symptoms.

Still, the outbreak continued. About a week after they developed symptoms, three people who went to the birthday party attended a church service. One churchgoer who had close contact with those three patients also developed COVID-19

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While the study provides a new window into exactly how cases spread, it may not show the complete picture, the study’s authors admitted. Only people who had COVID-19 symptoms were tested, so the size of the cluster might actually be bigger, the authors said.

It’s also difficult to say for certain where someone contracted the virus, according to the authors.

“Patient D3.1 was a health care professional, and, despite not seeing any patients with known COVID-19, might have acquired SARS-CoV-2 during clinical practice rather than through contact with members of this cluster,” the study said. “Similarly, other members of the cluster might have experienced community exposures to SARS-CoV-2, although these transmission events occurred before widespread community transmission of SARS-CoV-2 in Chicago.”

While more testing data is badly needed, the authors said, this study reinforces “the importance of adhering to current social distancing recommendations, including guidance to avoid any gatherings with persons from multiple households and following state or local stay-at-home orders.”