The hot springs and pools found in Yellowstone's thermal basins contain algae, bacteria and fungi that are unique throughout the world.
Caution is advised anytime of the year.
Following large animals tracks through geyser basins is no insurance of safety as many of these animals fall through the thin crust. Many elk, deer and bison winter in geyser basins. Winterkilled animals provide an important food source for grizzly and black bears emerging from winter dens. Check at a ranger station before you go exploring as some areas may be restricted. Don't approach or short cut through geyser basins after dark as there is a greater chance to accidentally slip or step into a hot spring.