Because when fighting with zombies, one might not notice if they've been scratched or bitten by them. There were cases where someone mutated only after returning to the base, hence the establishment of such a checkpoint.
If there are no injuries, one can enter the base directly. If there are injuries, one needs to stay in the quarantine room for 24 hours to confirm no mutation before entering the base.
Every seemingly unreasonable requirement is a rule added after real events happened.
Chu Jiangyue had stayed in the B City Safety Base in her previous life, so she was mentally prepared for this rule.
But it was the first time for Shen Zhigui and his group to come to the B City Safety Base, and hearing about physical inspections, they didn't look pleased.
"Why don't we send a message to Captain Song and let him take us inside?"
Shen Zhigui thought it was no big deal for them, as men, to be inspected, but it was quite inappropriate for a girl like Chu Jiangyue.
