Why do cancer patients get sick so easily?

Your wife's oncologist is absolutely correct: an ordinary cold virus can be life-threatening for people undergoing chemotherapy treatment for cancer. The overall goal of the cancer treatment *is* to help treat the cancer, but the challenge is that the medications that help target the tumor cells usually have negative side effects on the rest of the body. You and your wife are probably already far too familiar with many of the side effects of chemotherapy--fatigue, nausea, weight loss, aches and pains--and all of these things occur because the same drugs used to kill the cancer cells can also affect normal cells in the body. In particular, some of the normal cells that are most affected by chemotherapy drugs are the cells of the immune system. In many cancer patients, the cells that help us fight infections--and that keep us from catching every cold virus that's out there--are the first to be affected by chemotherapy and can drop to very, very low levels. In fact, certain kinds of chemotherapy can completely eliminate parts of the normal system. These cells will come back as the patient's bone marrow recovers from the effects of the chemo, but this can take some time. During this time, a person undergoing cancer treatments is effectively immunocomprised, ie they do not have fully functioning immune system.