In some cases, the audience has more information then Carol.
The virus works cross species. Did that rat get absorbed into the human hive, or does each species have their own, or can the virus make mammals do things against their control. Theres no proving that it didnt work on all of the rats and as a hive came up with a plan to infect the human by playing dead.
The virus has to be activated, as it was done all at once to the planet when it realized its time was up before being discovered and allowing people to get into shelters it wouldn't be able to access, killing the US government. Does that mean it can be deactivated and it worried that leaving scientists alive could reverse it.*
*Note my friend thinks they were dispersing the virus airborne via those planes, I think it was the US Government trying to get to nuclear shelters unaware they were already infected and the joining caused the planes to crash as they have only shown transmission via saliva.
)It not being able to lie may be a misdirect. It seems unlikely that it could get infected deep into restricted areas like nuclear subs across the planet without being duplicitous.
Have they shown any of the infected eating or drinking anything other the the "Milk" ? I thought about it this past episode and Id have to go back to when they had the meal outside with the other immune. If thats the case why pool the food supply if they are not going to use it and only use that drink.