how to nicely tell her she can’t see him?

my baby’s father & his mom have been seeing my son every two weeks on sunday. we don’t have any legal agreement or anything, just how we worked things out. he’s 7 months old and has recently developed a bad cough and is getting sick. is it fair to want to keep him at home? she has two young kids herself. how do i nicely tell her that she can’t take him this sunday when she asks?

yes i know it’a a dumb thing to ask advice on but i have bad social anxiety and am not good with these things. thank you.