Who were flagellants in Medieval Europe?

Question

What diseases did they catch from whipping themselves, how did it lead to their ultimate death?

Answer 1

A fanatical and heretical sect that flourished in the thirteenth and succeeding centuries, Their origin was at one time attributed to the missionary efforts of St. Anthony of Padua, in the cities of Northern Italy, early in the thirteenth century; but Lempp (Zeitschrift für Kirchengeschichte, XII, 435) has shown this to be unwarranted. Every important movement, however, has its forerunners, both in the idea out of which it grows and in specific acts of which it is a culmination. And, undoubtedly, the practice of self-flagellation, familiar to the folk as the ascetic custom of the more severe orders (such as the Camaldolese......http://www.newadvent.org/cathen/06089c.htm
flagellants (flăj'ulunts, flujel'unts) [key], term applied to the groups of Christians who practiced public flagellation as a penance. The practice supposedly grew out of the floggings administered as punishment to erring monks, although flagellation as a form of religious expression is an ancient usage. Among the flagellants it was an extreme expression of the ascetic ideal. Self-flagellation as a penance was approved by the early Christian church. However, the flagellant movement itself did not appear until the 13th cent., and it was not until c.1260 that the flagellants grew into large, organized bodies.
http://www.infoplease.com/ce6/society/A0818840.html

With the tide of monasticism came monks who whipped themselves or each other for their errors. One who was especially noted for this practice was Peter Damien, who hoped to suppress his lusts by scourging himself.

http://chi.gospelcom.net/DAILYF/2001/10/daily-10-20-2001.shtml

Answer 2

You have two excellent answers already. As to the medical aspect - "what diseases did they catch" - we can surmise
that wound infections with Staphylococcus and Streptococci would have taken a toll since there was little in the way of cleanliness and no antibiotic therapy to treat an infection.
If you add weakness from poor nutrition and blood loss, you have a debitated person who could easily succumb to overwhelming infection/sepsis.
{Sepsis means bacteria in the bloodstream which is often
fatal if untreated especially in immunocompromised hosts.}

Answer 3

They didn't catch diseases from whipping themselves. They felt that they needed to "play Christ" by whipping themselves for the sins of humanity as well as their own. They wore uncomfortable clothes (the whole suffering aspect) as well as whipped themselves.

They failed to note that only Christ Himself can erase the sins of humanity.

Answer 4

They were a religious sect during the Black Plague that whipped themselves to emulate the sufferings of Christ.