A new infection that simply makes people feel happy is treated as a threat by the authorities while its "victims" work to spread it to others.
Universal Pictures
Arachnophobia
Something that happened in quarantine
Bo Burnham: Inside
Attack of the Killer Tomatoes!
Gremlins 2: The New Batch
Dead Moon Rising
Hell Comes to Frogtown
Ebola Syndrome
Carry On Constable
The Simpsons Movie