LADUE, Mo. — Over four consecutive days in January, Margaret Hvatum ran a 5K, a 10K, a half-marathon, and a full marathon. The 70-year-old covered a combined distance that’s nearly equivalent to running the length of Manhattan four times.  By the end of the month, she was in a hospital bed. Hvatum, a part-time computer science professor, has a weakened immune system due to a rare condition known as primary immunodeficiency, which makes it difficult for her body to fight infections. Prior to her 2005 diagnosis, she had four bouts of shingles, a painful rash caused by a virus. For more...