SALT LAKE CITY — Nothing killed Mormon pioneers like cholera, which to them was a mysterious disease, and there isn't a close second.

A landmark new study on pioneer mortality indicates that cholera caused an astounding 40 percent of deaths on the Mormon trail. Tragically, historians believe pioneer habits unwittingly created cholera conditions that infected those who traveled later in the season.

Overall, the study found that pioneer travel was remarkably safe, with a mortality rate slightly higher than the national average, 3.5 to 2.9. But it also showed that "the later you left in the year, the more likely you were to die," researcher Aaron Smith said.

In fact, pioneer mortality rates rose every month through the summer.

The mortality rate for a pioneer company that left in April was 1.25, half of the average mortality rate in the general U.S. population in 1850.

Leave in August, however, and the mortality rate was above 5 percent, a four-fold increase and twice the national average.

That might seem obvious to those familiar with the stories of the Willie and Martin handcart companies, which left late in the summer of 1856, got caught in Wyoming snows and lost between 220 and 240 of the 1,165 people on the journey. But those tales are anomalies.

Cholera was the biggest killer and actually took a greater toll in late summer as the temperatures rose and rivers fell.

Compounding the problem was the fact that the cause of cholera was a mystery to the pioneers. They didn't know how to defend themselves from cholera bacteria, which secrete a toxin that inflames the large intestine, causing diarrhea and rapid dehydration. In severe cases, cholera can kill within hours.

"Cholera was a bad way to die, a really bad way to die," BYU-Idaho historian Andrea Radke-Moss said. "You could be walking around fine at breakfast, be contaminated at 10 a.m. and be dead by 3 p.m., especially if you were already weakened by dehydration."

In 1857, a decade after the Mormon exodus across the American plains began, the famous English medical journal Lancet reported that cholera was caused by bad air.

"In fact," said Dennis Tolley, the dean of BYU's statistics department, "cholera came from contaminated water. It was possible for pioneers to start on the path carrying cholera."

Radke-Moss said cholera worsened with the summer heat.

Pioneers regularly bathed and used as toilets the same places where they or others drew water for drinking and cooking.

"The pioneers traveled along big, slow-moving rivers like the Platte River and the Sweetwater," Radke-Moss said. "In the spring, they weren't bad, because they were swollen with snow melt and moved faster."

As temperatures rose, the rivers became better hosts for cholera, and the contamination could be lethal.

"The earlier you left in the spring, the more likely you were to find water that wasn't polluted by sewage," Radke-Moss said. "If you left later, by the time you reached the Platte and the Sweetwater, the more likely you were to find low, slow-flowing rivers in which hundreds of people have been defecating and urinating for three months."

Many pioneers were more lucky than good, regularly boiling water simply because they thought it tasted bad or because they could see organisms in it, Tolley said.

The mortality rate for Mormon pioneer companies dipped in September, but that's an anomaly. Only 2 percent of pioneer companies left that month, compared to the 60 percent that left in June or July and the 11 percent that left in August.

For one thing, said the statisticians who worked on the study, such a small sample size can skew results.

Smith, the researcher who worked on the study while a BYU undergraduate, said there is a good explanation for the relative late-season success of September companies: They were far smaller than most, so they could move faster.

The median size of a September-leaving Mormon pioneer company was 25, one-fifth the median size of companies that left between April and August.

"If they were going to leave that late," Smith said, "they would be planning to move fast."

The landmark study on pioneer mortality will be published in a coming issue of BYU Studies. The database was created over many years by a combination of church service missionaries overseen by retired LDS Church History librarian Melvin Bashore and BYU actuarial sciences students under Tolley's supervision.

Email: twalch@deseretnews.com