Should Have Known

Lucas was down with running nose and cough last week. Luckily, there was no fever. I was so busy with him that I forgot the virus is contagious. Then it hit me yesterday that Chloe is the easy target. When one child is sick, the rest in the family will eventually catch the bug. So, yesterday, when I got home after spending a few nights at my parents, both kids had bad running nose. Chloe couldn't sleep well. I was worried as this is the first time she got the bug. Had to take Chloe to see the paediatrician today, just in case it gets serious. Lucas running nose is attacking him again.
Now, when Lucas is sick next time, how to I 'protect' Chloe from catching the bug?