President Donald Trump and many other people have expressed optimism that the novel coronavirus may go away when weather warms in a similar way to the seasonal flu.

But it's summer in Australia, where the average temperature is about 74 degrees Fahrenheit, and at least 128 people there have gotten the coronavirus and three have died.

Tom Hanks confirmed Wednesday that he and his wife tested positive for the new coronavirus while in Australia's Gold Coast.

It's evidence that warmer weather doesn't make the coronavirus disappear.

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Some symptoms of the new coronavirus appear similar to those of the flu and common cold — COVID-19 is marked by fevers, coughing, and occasionally severe lung infections.

Because flus and colds tend to fluctuate with the seasons, retreating in summer and returning in the winter each year, many people — including President Donald Trump — have expressed optimism that impending springtime warmth could stymie the virus' spread.

But experts don't think we can bank on seeing a retreat of the new coronavirus, whose scientific name is SARS-CoV-2, come spring and summer.

"The short answer is that while we may expect modest declines in the contagiousness of SARS-CoV-2 in warmer, wetter weather, and perhaps with the closing of schools in temperate regions of the Northern Hemisphere, it is not reasonable to expect these declines alone to slow transmission enough to make a big dent," the epidemiologist Marc Lipsitch wrote in a post for Harvard University.

Lipsitch's point was underscored Wednesday by the news that Tom Hanks and his wife, Rita Wilson, tested positive for the coronavirus in Australia, where it's summer.

The coronavirus may not be seasonal yet

President Donald Trump at a meeting in the Oval Office on Thursday. AP Photo/Evan Vucci

In February, Trump issued a prediction about the coronavirus outbreak. "A lot of people think that goes away in April, with the heat that comes in," he said.

But Maria van Kerkhove, an infectious-disease expert with the World Health Organization, said at a press conference on March 5 that "we have no reason to believe that this virus would behave differently in different temperatures."

Hanks' diagnosis offered unfortunate evidence of that. The actor was filming in Australia's Gold Coast, where the average temperature is about 74 degrees Fahrenheit. So it seems the virus is still circulating in a part of the world experiencing summer. At least 128 people in Australia have been infected, and three people there have died.

According to Lipsitch, that could be because even seasonal infections can happen "out of season" when they are new.

"New viruses have a temporary but important advantage — few or no individuals in the population are immune to them," he wrote. "In simple terms, viruses that have been around for a long time can make a living — spread through the population — only when the conditions are the most favorable, in this case in winter," he said, referring to flu.

But never-before-seen viruses like the new coronavirus can "spread outside the normal season for their longer-established cousins," Lipsitch added.

Respiratory viruses are typically seasonal

Respiratory viruses can spread more easily in winter because cooler temperatures help harden a protective gel-like coating that surrounds the virus particles. A stronger shell allows them to survive long enough to travel from one person to the next.

The flu virus in particular "survives better in cool, dry temperatures," Amanda Simanek, an epidemiologist at the University of Wisconsin at Milwaukee, told Insider.

But of course, the Northern and Southern hemispheres don't experience the same seasons at the same time. So once China and the US see warmer weather, countries in South America and Oceania will be entering winter.

Plus, some countries don't experience dramatic seasonal changes at all, so "the flu circulates there year-round," Amesh Adalja, an infectious-disease expert at the Johns Hopkins Center for Health Security, previously told Business Insider.

There's still much to learn about how the new coronavirus will behave in the Northern Hemisphere come summer.

"We've only known about this virus for eight weeks or so — starting in late December, and now we're into March," van Kerkhove said. "So if anything, we don't know much about what this virus will do over the course of a season."