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.

Right. If there were bacterial spores, it would have still been infectious! Doc Shelly got it wrong. You have to heat it to 100 degrees Celsius to kill bacterial spores, not 90.

However, you don't know if it is viruses or something else.

You infect the mice and plants with fluid that wasn't heated.