Which would you expect to be the most active on a cold day?

Biologically thinking, I would say viruses.
Viruses are Inert organisms, they are not influenced by temperature on the opposite of other organisms like humans or bacteria (there are some exceptions in bacteria)
Temperature influences proteins, it could change their conformation and their activity, especially for enzymes.
Temperature decreases the energy flow, due to the decrease of thermal agitation of small molecules like ions in cells.
Viruses don't need active enzymes to live, it just needs host cells, which will be easier on cold days because of the decrease of immune system activity
On cold days you can see an emergence of viruses infections like flu or cold.

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