Like AussieBilly, I need a disease...

Food poisoning. Men will eat anything.

Bug bites, cougar attack, Indian attack, like Derek's idea of the bad liquor because it's more likely the women wouldn't have been allowed to have any, versus "here, I made the worst meal imaginable and I'm going to have all you menfolk eat it, but we girls are just gonna chew on some sand". Though I'm with Patti -- men and dogs WILL eat almost anything.

Taos Lightning would have been available at that time, and it was reputed to be able to strip rust off iron, paint from walls, and the lining from a man's stomach all at the same time. A good dose of the runs from that would certainly lay them up.

You could also combine that with a turn in the weather, which would delay things for a day or two.

You've probably got some pack animals right? A horse or donkey? You could have one of them attacked, or get sick. Or maybe the party just runs out of food and kills one. The pace would slow way down. Plus you could get some emotion out of that, often animals were so important to be part of the family.

Try this website
http://acc6.its.brooklyn.cuny.edu/~scintech/toilets/sewage_and_disease.html

Finding drinkable water along the trail, especially in the years that followed the original drive westward. Thousands and thousands of teams of oxen and mules had depleted the stores of water from viable streams, lakes, and springs. Those traveling throughout the late eighteen sixties would happen upon bodies of water known as 'Poison Spring' and 'Bad Water.' Many times the pioneers would walk through miles of badlands that contained no water whatsoever. The Great Basin was particularly difficult to pass for those who continued on westward; the dust would very often choke and blind the travelers as they made their way. Water could be found but was not drinkable; the teams often died from thirst or became so exhausted from the heat they had to be left to die.

Does it have to be an illness?

Marsha wanted to hold things up. In her story the party... "are within a week of their destination (Alburquerque--yes, with the original extra 'r'), and I need something to lay them low--an ailment or external force that will hold them up for several days but won't kill them."

*****
Hey, Marsha Ward ... does it have to be an illness? Could not one of the female members of the cast be having major birthing troubles? Infant mortality rates were not so very good back then. How about it being the wife/daughter of the party's leader, to give it more reason to stop for a day or so ... and then out of respect for the death of both mother and baby, another day or two -- funeral, reading of the bible, etc. Now you got a tear-jerker.
And you are correct with the extra 'r' in Duque de Alburquerque ... that was the original name given the community in 1706.

Dysentery.