If a houseguest shows up sick and you have a toddler in the house is there any way to prevent illness from spreading like a wildfire?

MIL showed up with a cold and played with LO for three hours yesterday before I got home from work. Help me think positively. I was so stoked for a healthy Christmas when daycare let out and everyone was healthy. My Christmas plans involve lots of pregnant women (including me) and squishy newborns.