weak immune system?

Have others of you had trouble getting over cold viruses?   I finished chemo and radiation treatments last July(2009) and since then have had three nasty cold viruses.  The latest one has lasted 5 weeks and still is ongoing.  I finally visited a doctor at a walk in clinic who checked me over,  ordered a chest x-ray and then gave me a prescription cough syrup but refused to give me an antibiotic.  I asked about the immune response and he suggested more exercise and certain foods that might help.  Is this a normal thing to happen and does anyone out there have any suggestions.  I