Remember that measles are extremely contagious in the community health setting...the public. In this case, with a band of young men sleeping and marching together. Even though we consider it an infrequent childhood disease now, it is particularly hard on adults.

Frequencies for modern cases from the CDC: "Measles itself is unpleasant, but the complications are dangerous. Six to 20 percent of the people who get the disease will get an ear infection, diarrhea, or even pneumonia. One out of 1000 people with measles will develop inflammation of the brain, and about one out of 1000 will die." So their cases may have very well been much higher. http://www.cdc.gov/nip/diseases/measles/faqs.htm

Evidently, there is also a subacute form that can take time to kill. It settles into the neurological system and the brain. http://www.ninds.nih.gov/disorders/subacute_panencephalitis/subacute_panencephalitis.htm

Also, I had found evidence from some literature in the Ft. Worth library several years ago that once they had been captured in AR, that they were taken aboard open boats up the Mississippi to Camp Butler. Some sick Union soldiers were placed on board, and more of the 6th Texas Infantry was exposed to another community illness. Can't remember what it was. But there was mention of them having to stop often on the sandy flats along the Mississippi to bury the dead along the river as they travelled.
N. Ripps