The great thing about climate change is that there’s always some new, horrifying consequence to worry about ― like ancient viruses and bacteria emerging from the ice as the Earth warms. Unfortunately, researchers fear we may see more of this in the future.

Some of these viruses and bacteria may have been trapped for millennia, and it’s not even totally clear yet what they are, let alone what kind of damage they might cause.

Researchers have encountered complex “giant viruses” with as many as thousands of genes in the melting permafrost of Siberia. One such virus, 30,000 years old, was still infectious when it was discovered in 2015, though it posed no danger to humans, Live Science reports.

There are also concerns that some old enemies thought vanquished could reappear. It turns out that permafrost is excellent at preserving bacteria and viruses that can lie dormant, then become reactivated with warming.

Scientists have discovered intact Spanish flu viruses in corpses buried in 1918 in the Alaskan tundra. When close to half of the population of a Siberian town in the 1890s died of smallpox, their corpses were buried in the permafrost along the Kolyma River. The banks of that river are now beginning to erode amid global warming, the BBC reports.

An apparent demonstration of the danger occurred in Siberia last August, when some 100 people (and 2,300 reindeer) were infected with anthrax in the first outbreak in the area since 1941. One boy died from the disease. Scientists believe the anthrax had been trapped in a long-frozen reindeer carcass that thawed during global warming and the particularly hot summer of 2016, releasing the bacterial spores into the environment.

“I have no words to express my feelings,” the region’s governor, Dmitry Kobylkin, told Agence France-Presse at the time. “The infection showed its cunning. Returning after 75 years, it took away a child’s life.”

Russian scientists expressed concerns in a 2011 paper about the danger of thawing permafrost leading to the resurrection of epidemics from centuries past.

And permafrost appears to be among the systems most vulnerable to global warming, note researchers in the journal Nature Climate Change.

But even in warmer climes, mounting temperatures can help grow and spread dangerous diseases, such the Zika virus, which has expanded its territory in a warming world.

Over a decade ago, researcher Paul Epstein predicted the possible spread of mosquito-borne illnesses as a result of climate change.

“Mosquitoes, which can carry many diseases, are very sensitive to temperature changes,” he wrote in a 2005 article in The New England Journal of Medicine. “Warming boosts their rates of reproduction and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse” ― all of which makes them more efficient at spreading disease.