This weekend is the unofficial end of summer, the season when, President Trump had promised, the weather would make covid-19 disappear, like a miracle.

Millions of Americans shared that hope and it’s easy to understand why: although many of history’s most dreaded epidemic diseases — cholera, polio, diphtheria — are at their worst in summer, the closest precedents for covid-19 seemed to be the influenza pandemic of 1918, the SARS pandemic of 2002 or the annual seasonal flu, all of which, as with so many respiratory infections, tend to fade away as summer arrives.

Yet the ever-rising number of covid-19 cases and deaths has revealed that the sun is no match for this new virus.

Trump is not the first to expect summer to bring medical miracles. Belief in the sun’s healing power is ancient, and while that faith seems rooted in sentiment rather than science, modern biomedicine has provided ample specific instances of how sunlight can deliver health benefits — from vitamin D to disinfection. Our efforts to exploit those benefits are not miraculous, but reflect science at work in its plodding, imperfect way. And so, while ultraviolet light has been some use in the fight against covid-19, deployed as a disinfectant on surfaces or in the air, sunlight is not the magic bullet that will end the pandemic.

Before the rise of modern biomedicine, many Americans embraced an emotional connection to the curative powers of the sun. “All the instincts of human nature,” one mid-19th-century writer asserted, “accept the idea of the healthfulness of light, and its agency in strengthening and cheering human life.”

But this embrace wasn’t a rejection of science: The same writer demonstrated her familiarity with the new science of electromagnetic waves, expressing wonder at the “five hundred millions of millions of vibrations a second” sunlight excited in the air. But her scientific claims rested on the old idea of the power of the sun to dispel miasmas — the bad smells caused by putrefying matter that resulted in disease. Sunlight, she assured readers, would cleanse those odors and banish disease, “whenever it rests on what is offensive or unclean, it is only to purify it by its chemical and subtle influences.”