During a recent walk in Fairmount Park, I passed a group of about 25 people enjoying a barbecue. Some were wearing masks, others were not, and none seemed to be social distancing.

It made me think about our individual choices, values, and the ongoing risk of COVID-19. Assessing risk is difficult and subject to all sorts of factors, including lack of data about the real risk involved as well as our perception of that risk. This is true whether it comes to assessing COVID-19 or decisions about skydiving, riding a motorcycle, or eating ice cream.

All decisions involve a balance between our estimate of the risk involved and the value that we will derive from the activity we are considering. Data matter when it comes to estimating risk. For example, some people do not swim in the ocean because they are afraid of a shark attack, but in fact data show that shark attacks are very rare and that driving to the beach is riskier than swimming in the water.

Although Pennsylvania has been very careful with regard to COVID-19, we still saw a peak in cases late last month, though cases have started to trend down again. But overall in the country, we are seeing the numbers for COVID-19 skyrocket. This means we need to be thoughtful about how we engage in what were previously normal activities.

I was taught a long time ago, during the HIV epidemic, that people often have a difficult time understanding the implications of their behavioral choices. One way to explain the risk of sexually transmitted disease from sleeping with multiple partners is to explain to patients that when they have sex with someone, the risk of contracting an infection from that encounter can be viewed as if they had sex both with the person they chose as well as everyone that person recently had sex with.

I learned that it can be helpful to ask patients to do the calculations themselves. You can ask, “If you have three sexual partners, and each of those individuals made a similar choice of having three partners, how many additional partners would it be like sleeping with?” Most people answer “nine” and typically you begin to see some discomfort in their eyes. Then you can ask them to calculate what the additional number would be if, the weekend before, each of those nine people had three partners (answer: 36). You can now usually see a tinge of surprise on their face. For most individuals, a light seems to go off in their mind as there is the realization that the choices they make, compounded with the choices those around them make, have very real implications on their risk.

Let’s apply that thinking to the barbecue. If each of the people at the barbecue also went to a barbecue the previous day with 25 other people, that would amount to an exposure to 625 people. We can then ask what the risk would be if the weekend before each of those people got together with 25 other people. That equates to an exposure over one week to 15,625 people.

We know that gatherings outside are safer than indoor gatherings, but there is still a good chance that some of those people have COVID-19 and will transmit it to others they have encountered. Some of those who get the infection will transmit it to friends and family without knowing it, some will get ill, some may need hospital care, and it is possible that one or more people may die as a result of those individual decisions. For the foreseeable future, our best choice may require being content with occasional contact with friends and family, and limiting the number of people at any one gathering while continuing to use masks and social distancing.

Neil Skolnik is a professor of family and community medicine at the Sidney Kimmel Medical College of Thomas Jefferson University and associate director in the family medicine residency program at Abington Jefferson Health.