Just weeks after returning to practice, sports leagues ranging from college football to professional hockey have suspended operations as several athletes and team personnel tested positive for the coronavirus. Twenty-three players and five staff members from the Clemson football program this past week were found to be infected, as were 13 football players at the University of Texas. Major League Baseball also suspended spring training after 40 players tested positive. The problem has not been for team sports alone; tennis star Novak Djokovic tested positive after organizing a charity tournament. And 16 NBA players have tested positive out of 302 tested just a few days ago. These examples show that sports risk being shut down again — with the cost being the health of prized athletes.

As states continue reopening, many people are anticipating the return of professional and college sports. But as of now, there are reasons to wonder whether they can start safely. Given how easily the virus can spread in high-contact sports, how it can spread beyond just athletes and the lasting aftereffects, it is clear that sports can’t return anytime soon without necessary safeguards.

Contact sports — such as basketball, football, hockey and soccer — involve close, face-to-face and prolonged contact with heavy breathing for several minutes at a time, which are key factors in spreading the virus. This is further complicated by the reality that players can be infectious but not have symptoms and are less likely to show symptoms at all by virtue of being younger, without other underlying health conditions. Even though it appears that the risk of transmission outdoors is far less than it is indoors, close contact still makes outdoor sports such as football and soccer incredibly risky.

And sports involve a wide range of people beyond athletes, too. There are coaches, trainers, staff, media and groundskeepers. Even if athletes are kept in a “bubble,” with minimal outside contact, as the NBA is proposing to do in Orlando, other staff involved with the games could inadvertently bring in the virus, especially given that testing is not perfect and prone to false negatives.

While we do not know the long-term effects of the virus, we do know that even some young people develop severe disease, requiring being put on a ventilator and, in a recent rare case, even a lung transplantation. But we do not fully understand why some do get very sick and others don’t. We also know that some people who have mild illness can still have symptoms and difficulty breathing for months. Even those who never develop symptoms show signs of lung inflammation on CT scan imaging, and it is still unclear whether there is lasting damage.

Beyond these obvious concerns, the nuances are relevant, too. Sports encompass more than the minutes on the field or the court — players spend extended periods indoors training and weightlifting, in team meetings, in locker rooms and in media sessions. This might explain why baseball and tennis players and even golfers have gotten infected.

Furthermore, all three seasons for America’s most popular sports — football, basketball and baseball — overlap to differing degrees with the flu season, which brings a feared dual epidemic of covid-19 and the flu this year.

It’s true that not all sports are created equal (full disclosure — we are both serious basketball fans). Indoor sports with sustained close contact and heavy breathing will be the least safe, while those that inherently require distance and are outdoors will be safer.

This creates a spectrum of safe sporting. Basketball, football and ice hockey are on the end of the spectrum of sports that are least safe for players. Soccer occupies a somewhat middle ground, while tennis and golf might account for the opposite end of the spectrum.

People want sports back, but we have a continuing and uncontrolled epidemic in this country, where countless citizens are vulnerable. But if sports do resume, serious steps will have to be taken to prevent the spread of the coronavirus.

The key one is driving community transmission down through public health measures, such as universal mask-wearing, testing, contact tracing, and isolating the sick. Countries that have done this successfully are in a much better position to resume sports than we are: South Korea is set to have real fans back in the stadiums for their baseball league as of this week. A number of European countries have resumed soccer leagues, although many without fans in place, and we have yet to see any publicly reported cases from these a few weeks out. But it’s important to remember that it was, in fact, a Champions League soccer match that may have set off a massive outbreak in Italy — and that the same could happen in any of these European countries.

Players and other personnel would have to be tested at least every few days given that a polymerase chain reaction test, the main test used to detect the genetic material of the virus, is a snapshot of just that moment. Pooling multiple tests into one run for scale could be a method to achieve this. There can and has been a notable false-negative rate of tests when someone may be infectious, particularly the days leading up to when symptoms start — which introduces a vulnerability in the system. False negatives could then increase the likelihood of transmission, but frequent testing could reduce these instances. However, the bigger picture here is that testing is still not adequate; prioritizing testing for athletes over sick patients and vulnerable communities makes little sense and may be unethical.

Another way to reduce the spread would be to ensure players limit physical contact with staff and management. Anyone who interacts with players should be part of their “bubbles,” meaning there are strict regulations on the numbers and types of contacts these people are allowed aside from with the players. Any off-court or off-field contact that they do have should employ risk-reduction strategies such as physical distancing when possible of six feet and consistently wearing masks without exception. But this would mean that crowded stadiums — or, really, any in-person fans — would be impossible, especially with many states having completely uncontained epidemics ongoing with high community transmission, such as Florida, where the NBA is looking to restart.

Even with these measures, there could still be unexpected circumstances like a false negative or an asymptomatic staff member. Someone could also break their bubble. Indoor facilities might have insufficient ventilation, which could make it easier for the virus to spread. Beyond this, if sports leagues need to be closed down again in the event of new cases, that has to be done quickly and safely.

If major league sports do start again this summer, it is entirely possible that they will have to be stopped once again. Too much is still unknown about covid-19, and as we reopen, we are taught where our public health response’s weaknesses are. Sports may be a welcome return to some sense of normalcy, but a lot more than a game can be lost if fans, teams, athletes and others give in to wishful thinking rather than realism.