According to New Scientist, human activity and prior attempts to kill the fungus have made the ecology of Lascaux similar to a hospital cooling tower.
The team conclude that a benzalkonium chloride spray applied between 2001 and 2004 to kill the fungus is to blame, as it allowed bacteria brought in by human visitors to thrive (Naturwissenschaften, DOI: 10.1007/s00114-009-0540-y). "It produced a drastic change in the cave biodiversity," says [Cesareo[ Saiz-Jimenez.
The report mentions that the cave ecology now includes pathogens linked to disease outbreaks in humans. That seems like a good reason to stay out. Maybe too good. Like that part of Close Encounters where the army scatters dead cattle all over Wyoming.