COVID-19: What happened here? CDC says four factors fueled unchecked spread . . .

But agency notes continued testing, knowledge gaps . . .

In late February, the United States was home to just 53 people who had been diagnosed with COVID-19.

That month, the U.S. Centers for Disease Control and Prevention had separated categories of COVID-19 case counts in a way the agency hoped would accurately reflect risks to Americans. The numbers of people stricken were broken down into two smaller numbers: 14 people who had been diagnosed with COVID-19 by public health departments in the United States, and an additional 39 people who had been returned to the United States from high-exposure areas and diagnosed soon after arrival, while quarantined. All had either spent time in an area where the novel coronavirus already was spreading locally and unchecked, or had been in contact with someone who had.

One week later, a series of reports -- of people diagnosed who had no recent international travel, or exposure to people who did --  indicated, a CDC report released Friday says, "the initiation of pandemic spread in the United States." By mid-March, people who had become infected with COVID-19 began to overwhelm first health facilities and then morgues, prompting the start of a series of changes that would lead to cancellations of gatherings from business meetings to religious rites, from arts events to sporting competitions, upending commerce, culture and communities in the process.

By April 21, the CDC account, in the agency's Morbidity and Mortality Weekly Report notes, the great majority of the then nearly 800,000 people diagnosed had been exposed to the virus because of widespread transmission in their communities.

Four factors, the report suggests, led to the explosion of transmission from a carefully traced and documented 53 people, to the acknowledgement now that any figure given for the numbers of people in America diagnosed, infected with, recovering from and spreading the virus -- more than a million as of the first week of May -- is an underestimate of still unknown proportions.

The continued arrival of travelers from high-exposure places, was one of the factors, according to the report. It notes that during February more than 139,00 travelers from Italy arrived, along with more than 1.7 million people from European countries whose citizens can cross neighboring borders without passports. More than 100 people from nine Nile River cruise vacations tested positive for the virus after they returned to 18 states across America.

Large and still unrestricted gatherings in February also helped distribute the virus across the country, according to the report. While Mardi Gras festivities in New Orleans in the waning days of February drew more than a million attendees, in early March a biotechnology company's annual meeting drew about 175 international attendees. (Days after that conference ended, the annual Conference on Retroviruses and Opportunistic Infections  -- set to draw about 4,000 attendees from around the world in two days time, advised attendees to stay home, or if on the way, go home, and switched to a virtual venue).

By then the discovery of the first infections in a nursing home served as harbinger of the obvious -- that facilities that are home to the most vulnerable populations, as well as other densely crowded settings home to people already facing challenges to health care access and safe working conditions -- would allow the virus to gain speed in its spread.

Finally, the report cites "challenges" to monitoring the spread of the virus -- because the widespread symptoms seen during the continuing flu season obscured the surfacing of symptoms of COVID-19 infection, and because people with mild or no symptoms also were infectious -- but also because of inadequate testing access.

Of all the factors, the last remains one of the limitations of the report's conclusions the authors note, along with continuing gaps in knowledge of the actual number of COVID-19 cases the United States has seen so far.