As I discovered shortly after posting, I was infected with the same ailment as D1, although with a different manifestation. We spent Christmas at the trailer formerly occupied by my late grandmother, trying to avoid contaminating various relatives and trying to get D1 to keep something down, the latter of which did not occur with any degree of success until late afternoon.
In the evening, DOB determined he would sit up with D1 as long as possible and let me get some sleep. About 3 a.m. he awakened me and traded places; within an hour or two it was evident that it was not just his turn to sleep, but his turn to be sick.
Fortunately D1 and I seem to be doing much better, so it appears to be relatively short-lived. It is frustrating to be infected with a most unfestive disease on Christmas, and a most anti-social disease on the one week out of the year one has to visit half one's friends and relations.