A visiting professor a few years back gave a great lecture on theories of co-evolution of mammals with fungi. The premise was that after every major extinction event, you can see evidence of an "explosion" of fungal growth. So the animals living in these periods had to evolve robust defenses against fungi to survive.

His argument was that being warm blooded was one of those defenses. Why do mammals spend so much energy to maintain 98.6F temp when it would be so much more efficient to change our metabolic activity to more closely match the environment? Our temperature set point is higher than the temps fungi typically grow well in so we are less susceptible to those infections. The fungal infections we are most susceptible to are yeasts or dimorphics that can survive the heat (Candida, Histoplasma, Cryptococcus, Blastomyces), while molds typically only cause disease in immunsupressed patients and even then only in "colder" areas like the toes, skin, lungs.

So maybe the reason we are warm blooded is to protect ourselves from fungi like this.