refers to the use of medications that are designed to help destroy cancerous cells within the body. Unfortunately, several chemotherapy agents are not completely specific for cancerous cells, and end up having adverse side effects due to their effect on the body's healthy cells. This often includes damage to the cells of the immune system, as well as the cells in the bone marrow (where the body's new cells are produced).
Patients will often be immunosuppressed after a round of chemotherapy, meaning that their levels of infection-fighting cells are lower than normal, making them more susceptible to infection. If an infection is contracted, it will be more difficult for the body to fight it off than it would be otherwise. Your mother may have contracted a common cold virus that she normally would have fought off within a few days. In the absence of a normal immune system, however, her body may have allowed the virus to wreak more havoc than usual, including spreading to the lungs when it would normally just cause sinus congestion.