The Field of Dreams park in Unity is closed during the COVID-19 pandemic. Photo courtesy of Dana Wilde

The backyard is safe. The woods surrounding our house are safe. (Until the ticks get fully active, anyway.) Walks around the Unity park seemed safe, keeping our distance from humans. Something we woods-dwellers sort of do anyway. Then the park closed. Probably a good move.

When will it be safe to go to the grocery store again, to play basketball, to go back to the office or shop?

COVID-19, we hear, could subside in a month or two and “go away,” as certain wishful thinking has gone. Or it could subside and then come back in the fall. Or it could stay around all summer, and into next fall and winter. Of course, we hope it will “run its course.” But what does that mean?

Here’s what I’ve found out about what the epidemiologists think “flu season” means, and why they’re not sure whether COVID-19 will conform to it. Much of this is condensed from an article by Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard.

The coronavirus is a new strain of viruses that cause colds and flu. This new virus is named SARS-CoV-2. It causes an illness called COVID-19. There isn’t enough information yet to peg exactly, but its fatality rate appears to be between about 1.5% and 4% of the people who get it. The average fatality rate for the seasonal flus seen each year is around 0.1%. The fatality rate for SARS was around 10%, for MERS around 35%, and for avian flu, upwards of 50%.

Since COVID-19 causes deaths at a significantly lower rate than its cousins SARS, MERS and avian flu, why has it already killed so many more people than those others? The answer involves mainly how the diseases are transmitted. Simply put, more people are contracting the COVID-19 virus because so many people are transmitting it. Even people with no symptoms are transmitting the virus through coughs, sneezes and touching surfaces where the virus can remain viable for hours or even a couple of days.

With SARS, MERS and avian flu, fewer people were spreading the disease, so fewer people were getting sick, and therefore fewer died, even though the symptoms were usually more severe. The avian flu virus is transmitted to humans from birds, rarely between humans. MERS is transmitted through close contact, and is much more easily contained than COVID-19. The severity of SARS symptoms allowed public health officials to quickly identify the sick who could transmit the disease, and to take aggressive measures that stifled a potential pandemic. COVID-19 can be transmitted before you even know you have it.

Is COVID-19 going to subside in warm weather, as other flus tend to do? Maybe. Maybe not. The epidemiologists think four factors are involved in “flu season.”

First, it seems clear — but is not certain — that humidity limits the transmission of viruses. It could be that airborne viruses simply can’t travel as far in humid air because they drop to the ground faster in the heavier water vapor before they reach a host. Warmer months are more humid, colder months are drier, so viruses may travel better in the drier air of fall and winter. It’s also possible that when your nose, for example, dries out in winter, the mucuses that normally form a line of defense against infection are less effective. But they don’t go away in summer. They’re still around.

A second factor is probably that in cold weather, humans are packed more closely together every day. It may well be no coincidence that the resumption of flu season in the fall more or less coincides with the resumption of school. In closer quarters, flu viruses spread more readily.

Another factor could be that the number of individuals who are susceptible to the disease is cyclical. In other words, the virus infects hosts that do not have immunity to it. By spring, more people are immune, and so fewer individuals are available for the virus to infect. About that time, people are getting back outdoors more, so the possibility of transmission is further reduced. Then as the weather gets colder and people start packing inside again, the virus finds more available hosts and the cycle begins again.

It may also be that the lack of sunlight in winter weakens the overall working of the immune system. In particular, sunlight provides vitamin D, which has been shown to be a factor in fighting respiratory illness. This is far from certain, though.

These are all hypotheses. They may play into an overall explanation for why colds and flu tend to increase in cold weather. Some may be more important than others. Some may turn out to be of negligible importance.

So COVID-19 may or may not “go away” within a month or two. Before this winter, no one had immunity to the illness. It is not known if there is even such a thing as immunity to it. We won’t know until we cycle through this and find out who gets sick in the next round and who doesn’t. It is not known for sure whether a vaccine will work. Going by the other viruses, it seems likely one will work. That’s the hope, anyway.

The pandemic could subside during the summer. There could be a vaccine we can all get. And then it could be safe to go out again. But get yourself ready mentally. This could be a really long haul.

Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His recent book is “Summer to Fall: Notes and Numina from the Maine Woods,” available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.