Help older child is sick

Yas

What do I do my 4 year old is sick with a low grade fever a cough and sore throat I have no clue where he got it from we weren’t even around anyone and no one I know is sick with anything 😩 how do I protect baby from catching whatever this is?? What are symptoms for rsv, corona, we already had flu A during my pregnancy so we’re immune to that I guess.