While public discourse around college football's return has focused on the student-athlete risk, fan attendance could have a greater impact on overall public health. For the programs still planning to play this fall, yet another decision looms: Will there be fans at games? And if so, how many?

There have already been instances of the health risks that come from crowded sporting events. The first came Feb. 19, as nearly 2,500 fans of Spanish soccer club Valencia traveled to Italy to watch a Champions League match against Atalanta. They were among the 44,236 fans inside Milan's San Siro Stadium, and several weeks later, after it was determined to be a coronavirus "super-spreading event" that played a significant role in outbreaks locally and in Spain, the match became known as "Game Zero."

ESPN interviewed more than a dozen epidemiologists and infectious disease experts about the role college football could play in COVID-19's spread, even with significantly reduced fan capacity. To illustrate fan movement after Saturday games, ESPN mapped out anonymized cellphone tracking data provided by X-Mode Social, a data visualization company. We selected three 2019 matchups -- Ohio State at Nebraska, Stanford at USC and Ole Miss at Alabama -- to show how fan dispersal differs by region, and how far or concentrated the potential area of exposure could be, if or when crowds gather again in the fall. In each case, the available data represents no more than 11.9% of the game's reported attendance.

The map begins six hours after kickoff of each game and updates in six-hour increments over the subsequent 24 hours to show which counties fans returned to. The darker shade of red indicates how more densely populated a county is with fans that attended the game.

Six hours after kickoff: After the No. 5 Buckeyes routed the Cornhuskers 48-7 in front of the 371st consecutive sellout crowd at Memorial Stadium, fans have scattered to more than 30 counties in the surrounding region.

18 hours after kickoff: Based on location data collected from 8,955 cellphones in attendance, fans returned to cities as far away as Denver and Chicago.

24 hours after kickoff: Fans' travel was traced to places as far away as South Florida, Southern California, Oregon and Connecticut, but most returned to Lincoln's surrounding communities. This process of repeatedly mixing and spreading is what epidemiologists said is potentially most problematic when it comes to welcoming fans back to stadiums, even with limited capacities.

The risks of crowds and COVID: "I know people like to call them mass gatherings, but for an epidemiologist, these are mass-mixing events. You essentially come in, a handful of people who may be infected get to mix with more individuals than they would otherwise mix with, who can then go back to multiple places. The other dynamic that comes into mass-mixing events is something called super-spreading events, and super-spreading events are where an individual is very efficient at transmitting not just to the usual two to three people we see with this disease, but potentially 15 to 20 people." -- Dr. Ali S. Khan, dean of the College of Public Health at the University of Nebraska Medical Center

B1G crowds avoided: The Big Ten, which postponed its 2020 football season on Aug. 11, ranked second nationally in conference attendance with a combined 6,311,298 fans attending 97 games (65,065 average).

AP Photo/Nati Harnik

Why the Big Ten postponed its season: After backlash from fans, players, coaches and some universities (including Nebraska), Big Ten commissioner Kevin Warren put out a letter attributing the league presidents' decision to "too much medical uncertainty and too many unknown health risks regarding SARS-CoV-2 infection" and its impact on the student-athletes.

Six hours after kickoff: The announced attendance of 62,109 was just 81% capacity at the newly renovated Los Angeles Memorial Coliseum (76,750). Data included 1,868 cellphones, roughly 3% of the reported attendance.

18 hours after kickoff: There are 157 cities in California with larger populations than USC's average crowd in 2019 (59,358), its smallest turnout since 1987. Coming off their first losing season since 2000, the Trojans entered their Pac-12 opener unranked after losing starting quarterback J.T. Daniels for the season in Week 1.

Meg Oliphant/Getty Images

24 hours after kickoff: By 7:30 p.m. the next day, the large majority of the phones tracked remained in Los Angeles or its neighboring counties.

The danger of traveling crowds: "Travel patterns [in college football] would be just like for any work or academic conference that brings people from all over the country, or even all over the world. The more people you add and the further afield from which they come, the higher the danger is." -- Dr. Zachary Binney, Emory University epidemiologist

Los Angeles by the numbers: Los Angeles County has over 10 million residents (more than 41 individual U.S. States) and 239,756 confirmed COVID-19 cases through Aug. 29, according to Johns Hopkins University of Medicine.

Reaction to the Pac-12's postponement: When the conference announced on Aug. 11 it was postponing the season, county health guidelines still had not permitted USC and rival UCLA to practice. USC athletic director Mike Bohn was supportive of the decision, despite expecting it to cost upward of $60 million in lost revenue. "In my view, no reasonable-minded individual could have listened to the facts presented by our medical experts and believed that we had any other option at this time," he said.

Six hours after kickoff: With Oxford, Mississippi, just 170 miles from Tuscaloosa, it's an easy trip for opposing fans. After No. 2 Alabama cruised to a 59-31 victory in the SEC season opener, fans were traced to three neighboring states: Tennessee, Georgia and Mississippi.

18 hours after kickoff: Data collected from 11,863 cellphones showed fans dispersing back to their communities in all parts of Alabama and neighboring states by 8:30 a.m. the following morning.

24 hours after kickoff: According to the collected data, fans did not travel beyond their indicated locations at the 18-hour point.

One conference's protocol for 2020: On Aug. 18, the SEC released fan guidelines for attending games. Each school is set to determine how many fans it will allow in accordance with state and local guidelines. At Alabama, the plan is to allow approximately 20% of the normal operating capacity, which equates to roughly 20,000 people. The school said it will implement other measures, like the requirement of face coverings and physical distancing where lines form, but enforcement of those policies is not yet clear. Alabama will not allow tailgating, will require contactless mobile tickets, and will require fans, using the honor system, to answer questions about potential COVID-19 symptoms and exposure in a mobile app beforehand.

How to protect fans in 2020: "[Allowing] as few people as possible is one first step to opening up and then, maybe opening up with only a proportion of the seats available for sale in the stadiums so people can be spaced out. You have like one group here and another family group 10 feet away so you get a bit more spaced out. [Going from that to] sort of gradually going back up to a full stadium, I think would be the ideal from a sort of epidemiological perspective." -- Dr. Eleanor J. Murray, assistant professor of epidemiology at Boston University's School of Public Health

Nick Saban urges mask use: On Aug. 19, Alabama coach Nick Saban stressed the need to wear masks and follow health guidelines. "A lot of people have asked that we wear masks when we're in public, when we're in crowds, when we're in large groups of people, that we keep social distanced. I don't think they're doing that just for the heck of it. I think there's a reason for it. We're trying to control the spread of this disease, and I think that our ability to do that is going to go a long way in saying whether we can play football or not." Alabama recorded 1,043 positive tests among students, faculty and staff since between Aug. 19-27, according to data released by the school.

AP Photo/Kent Gidley, University of Alabama

Why college crowds are unique: The 15 largest-capacity stadiums in the United States exist primarily for college football and, in a typical year, the general population's access to attending a college game is unlike anything that exists in professional sports. College football is sanctioned by the NCAA in every state except Alaska, with the 130 FBS programs illustrated by red markers here. Those teams, spanning from FBS to Division III, drew a total attendance of more than 47.5 million last season. Even with fewer teams in action and limited-capacity crowds, the prospect that college football could play a role in spreading the coronavirus is too obvious to ignore.

Expect the unexpected: Comparatively, this map now shows which areas are postponing (in gray) and the others in which 76 teams are moving forward with plans to play (red). Through Aug. 30, four FBS conferences (Big Ten, Pac-12, Mid-American and Mountain West), plus independents UConn, Massachusetts and New Mexico State, have announced they will not play this fall. Last year, those schools combined to draw just shy of 13 million fans, which accounted for 38% of the total FBS attendance in games at home sites. It remains to be seen what college football will look like in the fall, but it will be under conditions unlike anything the sport has ever seen.