West Hartford, Conn.

I HAVE chronic lymphocytic leukemia. Three months ago, I underwent an allogeneic stem-cell transplant, in which my wise, 52-year-old white blood cells were replaced by bewildered, low-functioning cells from an anonymous European donor. For the next seven months or so, until those cells mature, I have a newborn’s immunity; I am prey to illnesses like chickenpox, the measles and the flu.

These diseases are rarely fatal, unless you’re a newborn or someone with a suppressed immune system like me. My newborn buddies and I do have some protection, however: the rest of you.

Young babies, the immuno-compromised and people who get chemotherapy are not able to process most vaccinations. Live vaccines in particular, like those for measles and chickenpox, can make us sick. But if 75 percent to 95 percent of the population around us is vaccinated for a particular disease, the rest are protected through what is called herd immunity. In other words, your measles vaccine protects me against the measles.

It’s the reasoning of Clarence, the angel from “It’s a Wonderful Life”: If you are vaccinated, you won’t pass a disease on to someone else, who won’t pass it on to six more people, and on and on. To quote Clarence, “Strange, isn’t it? Each man’s life touches so many other lives.”