Bats, a symbol of Halloween, may be responsible for canceling it this year.
The coronavirus that has grounded trick-or-treaters likely came from bats.
These creatures of the night have evolved a spooky ability to harbor a number of viruses that can kill humans -- without getting sick themselves.
How they do it may hold the key to immortality -- or a longer life, anyway.
Guilt by association
Though there is no smoking gun showing that the coronavirus causing the COVID-19 pandemic came from bats, the virus is closely related to several others they harbor.
Bats also are known to carry rabies and the Marburg hemorrhagic fever virus, and they are lead suspects as the source of Ebola and the 2003 severe acute respiratory syndrome, or SARS.
It sure seems like they carry a lot of nasty viruses.
But "maybe we just have a lot of bat viruses because there's lots of bats," said University of Glasgow researcher Daniel Streicker.
There are roughly 1,400 different species around the world, Streicker noted, second only to rodents, which also carry a lot of diseases.
"It isn't the bats. They figured out how to deal with their viruses," University of Saskatchewan microbiologist Vikram Misra said.
Bats also do a lot of good. Bug-eating bats provide an estimated $3.7 billion in insect control services. They pollinate and disperse seeds for many plant species, including the tequila-making blue agave.
Their virus-resisting powers may be an unexpected byproduct of evolving to fly.
Flying requires a tremendous amount of energy. Generating that energy also produces toxic byproducts that can damage cells.
Normally, cell damage would trigger inflammation, the immune system's first line of defense. The same inflammatory response kicks in whether the damage comes from toxic molecules, injury or infection. As part of the response, the body mobilizes cells to the damaged area that can blast germs or infected cells.
Too much inflammation can kill. Overactive inflammatory responses are what lead to lung damage, blood clots and other fatal complications in COVID-19 patients.
"Maybe bats had to down-regulate their responses just not to get inflamed every time they had to fly," said University of Rochester biology professor Vera Gorbunova.
But flight "doesn't explain everything about bats," she said. Another reason their immune systems are different from most mammals may be because of the way they live.
Bats live in colonies that can number in the millions of individuals, roosting shoulder to shoulder. Diseases could spread quickly in those close quarters.
"They probably evolved defenses because they're exposed to a lot of viruses," Gorbunova said.
For whatever reason, their adaptations appear to be so important that they evolved independently in different bat species, a new study shows.
Turning down a key immune response would seem to leave bats open to infection. But evolution has turned up another line of defense that targets viruses.
Bats and viruses may have reached a "wonderfully balanced relationship where viruses don't cause diseases and bats don’t get rid of the viruses," Misra said. He and his colleagues at the University of Saskatchewan have found that bat cells can remain infected for months.
But stress -- from humans encroaching on their habitat or capturing them to sell at live-animal markets -- may disrupt that relationship.
"If you upset this delicate balance in such a way that the viruses now have an upper hand," Misra said, "then the viruses start to multiply and the bats now start to shed more of these viruses. We think that that may be one of the reasons why spillovers occur" and the viruses jump into another species.
"We can't say for sure that that's the case," he added, but he and his colleagues are testing the idea now.
Live long and prosper
Aside from reaching a detente with viruses, bats may have reaped another unintended reward from learning to fly. They may have discovered the fountain of youth.
Bats live disproportionately long and healthy lives for their size. Take North American little brown bats, which are "about the size your thumb," Misra said.
"Normally, you would expect them to live maybe two years, three years, if you compare them to animals that are of comparable size," he said. "These bats live 30 or 40 years."
The key may be their ability to tamp down inflammation without leaving themselves exposed to viruses.
"Inflammation may be the driving force of age-related diseases," biology professor Gorbunova said. It is a factor in Alzheimer's disease, some forms of heart disease, diabetes and many others.
With more research, she added, perhaps the bats that seem to be responsible for so much suffering can someday help us live longer.