DEAR DR. ROACH: I am a 58-year-old male. Several months ago, I lost 17 pounds, and even though I have gained back about 2 pounds in the past month, I cannot gain the weight back, even by increasing my food intake. I feel tired all the time, and my energy level is not good.

My doctor did a CBC, and my white blood cell count is low, at 3.1 (range is 4-11). He asked me if I recently had a cold, and I told him I had not.

Since it was a new doctor, there are no previous results for comparison. The doctor did not tell me anything or instruct me to do anything else, which concerns me. There are two other results with low readings: lymphocytes are 0.1 (range is 1.2-3.4), and lymphocyte percent is 3.4 (range is 20-45).

I would like to know what, if anything, I should do next, other than waiting a few months for another blood test. Do I have to be worried about this? — V.B.

A: I am sorry, but you do have to be worried about this. Your lymphocyte count is extremely low. There are many potential causes, some of which are serious. Infections are the most common cause. Any serious infection can temporarily reduce lymphocyte count, but most of these are acute, to the point that people need hospitalization.

One common infection to cause a low lymphocyte count is HIV.

In the early days of the epidemic, a low lymphocyte count was a clue to either acute or advanced HIV. The Centers for Disease Control and Prevention recommends, and I agree, that all adults should get an HIV test as a part of their routine checkups, and that people with ongoing risk should get a test annually.

A partial list of other causes for low lymphocyte count include medications (usually powerful anti-immune-system medicines or chemotherapy), autoimmune diseases such as lupus or rheumatoid arthritis, lymphoma and other cancers, and primary diseases of the immune system, such as aplastic anemia.

Previous results would help indicate whether this is a new condition.

This can’t wait a few months for a follow-up test. If your doctor isn’t looking aggressively for the cause, you need a second opinion.

DEAR DR. ROACH: While reading your recent article about sulfites, a question came to mind: What are sulfites? Are they mineral, chemical, organic? Do they grow on trees? How do they affect food? — M.C.

A: Sulfites are chemicals found naturally at low levels in wine and some foods, and are used as preservatives at significantly higher levels.

Most people are not sensitive to sulfites even at high levels, but some people have significant reactions even at fairly low levels. People with severe allergies need to know that sulfites are not always indicated on the label and should educate themselves about all the foods containing sulfites. One resource I found is at edis.ifas.ufl.edu/fy731 online.

Everything we eat, drink and breathe is composed of chemicals. The difference between a “bad” chemical and a “good” one is one of dosage: We can’t live without oxygen, but too much oxygen is toxic. Most medications are useless at extremely low doses, helpful for some people with certain conditions at the proper dosage, and toxic if taken at too high a dose.

Sulfites are no exception to this rule and, like any other chemical, some people are more sensitive to toxic effects at lower doses than others.

DEAR DR. ROACH: I have never had the shingles, nor chickenpox. I couldn’t find a record of it, even in my baby book. Do I need to have the shingles shot? I am over 70 and take several medications for high blood pressure and cholesterol. — C.S.

A: I recommend the shingles vaccine to someone in your situation, despite the fact that the shingles vaccine isn’t perfect. In the initial trial that got the vaccine approved, following almost 40,000 adults over 60, 3.3 percent who did not receive the vaccine got shingles in three years, and 1.6 percent of those who received the vaccine developed shingles.

However, for the dreaded complication of post-herpetic neuralgia, having the vaccine reduced the risk from 0.6 percent to 0.2 percent in people over 70.

Most people over 70 have had chickenpox. Sometimes the disease is so mild that it can go unrecognized. But both people who have and have not had chickenpox should get the vaccine.

People with conditions weakening their immune system should not get the vaccine.

The absolute benefits of 1.7 percent reduction in developing shingles, plus the 0.4 percent reduction in post-herpetic neuralgia are not very large. About 50 people would need to be vaccinated to prevent one bad outcome in three years. Over a long time, however, the absolute benefit is likely to get more impressive.

More importantly, the risk of the vaccine is small. The major adverse events have been headache and sore arm. In my opinion, the benefits far outweigh the risks, and by vaccinating a lot of people, some cases of shingles and post-herpetic neuralgia can be prevented.

Having seen how devastating post-herpetic neuralgia can be in an older person, I think it is worth it.

DEAR DR. ROACH: Can exposure to infectious mononucleosis trigger or cause rheumatoid arthritis to flare up and become active? I would appreciate your comments. Thank you. — J.C.

A: The Epstein-Barr virus —the cause of infectious mononucleosis (although there are other germs that can cause a similar picture) — has been associated with rheumatoid arthritis, and there is some evidence that EBV may be a trigger that causes RA and possibly other autoimmune diseases, such as lupus, to become active in a person who is genetically susceptible.

However, this evidence is only speculative at this point, since an alternative explanation for the association is that the immune deficiencies in RA and other autoimmune diseases allow for abnormal persistent replication of EBV.

What is more important is that nearly all adults (90-95 percent) have had EBV infection, and re-exposure to the virus does not cause clinical infection and almost certainly will not trigger new onset or a flare-up of RA in people who already have been exposed to EBV.

Dr. Keith Roach is a physician at Weill Cornell Medical College and New York Presbyterian Hospital. Email him questions at ToYourGoodHealth@med.cornell.edu. Consult with your physician before making changes to your personal regimen.