Just to add: once they get an infection, their immune systems can't fight off the illness like we can.
If a person with cancer is undergoing chemotherapy, their immune system is quite vulnerable because of the drugs that they are given. The chemo not only kills the tumor cells, they kill the good cells including the cells that make up the immune system. This makes them open to many infections, some life threatening. A simple cold can easily become pneumonia in these people. That is why it is important to stay away from them if you have any symptoms of an infection because they can get so sick very quickly.
I hope that this helps..Sunny