Chemotherapy is the mode of treatment in which medicines which can kill the cancer cells are administered to the patient's body.
These medicines target the actively dividing cancer cells. At the same time, they may also affect the normal cells in blood, bone marrow, etc.
Therefore, during the course of treatment, as the bone marrow gets damaged, it temporarily produces less lymphocytes.
Since the lymphocytes decreases, the body becomes more vulnerable to infection.