Dr. Begely: Ok, let's infect the mice and plants now.
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Dr. Shelly: I concur.
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You infect the mice and plants.

Dr. Begely: Hum… neither the mice or the plants got an infection… that means it must be a virus.

Dr. Shelly: No! There could have been bacterial spores in the fluid. They are killed by the heat, but could pass through filters, and survive alcohol, and formalin!

Bacterial spores? Or no?

 I think it's definitely viruses. Doc Shelly is wrong.

 It's definitely not a bacterial spore, but could be a virus or something else. We should try to see if the liquid was infectious before we heated it.

 Doc Shelly is right. There could be bacterial spores in the fluid.