From the article I linked above:
Quote:Two years later, scientists managed to revive an 8-million-year-old bacterium that had been lying dormant in ice, beneath the surface of a glacier in the Beacon and Mullins valleys of Antarctica. In the same study, bacteria were also revived from ice that was over 100,000 years old.
However, not all bacteria can come back to life after being frozen in permafrost. Anthrax bacteria can do so because they form spores, which are extremely hardy and can survive frozen for longer than a century.
Other bacteria that can form spores, and so could survive in permafrost, include tetanus and Clostridium botulinum, the pathogen responsible for botulism: a rare illness that can cause paralysis and even prove fatal. Some fungi can also survive in permafrost for a long time.
Some viruses can also survive for lengthy periods.
In a 2014 study, a team led by Claverie revived two viruses that had been trapped in Siberian permafrost for 30,000 years. Known as Pithovirus sibericum and Mollivirus sibericum, they are both "giant viruses", because unlike most viruses they are so big they can be seen under a regular microscope. They were discovered 100ft underground in coastal tundra.
Once they were revived, the viruses quickly became infectious. Fortunately for us, these particular viruses only infect single-celled amoebas. Still, the study suggests that other viruses, which really could infect humans, might be revived in the same way.
So botulism could cause problems.