It's increasingly clear that a lot of the spread is occuring through presymptomatic people, folks that are in the incubation period, many of whom are not at high risk themselves.
The reason this is a standard of practice is because… measles has a long incubation period, meaning it can take a while for you to be exposed and have symptoms.
In general, infected people are most contagious starting from the final day of the incubation period through the prodromal and exanthem phase, which roughly works out to be 4 days before to 4 days after the onset of the rash.
And also in fact, scientists have found that there's a 17 day latency period between a national tragedy happening and you being able to make a good joke about it.
And while the incubation time for V. cholerae can take hours, to 2-3 days; severe dehydration and depletion of electrolytes can happen within 4 to 12 hours of the first bout of diarrhea or vomiting.