Conversely, researchers have hypothesised that a precursor to syphilis may have been carried from the New World to Europe after Columbus's voyages. The findings suggested Europeans could have carried the nonvenereal tropical bacteria home, where the organisms may have mutated into a more deadly form in the different conditions of Europe. The disease was more frequently fatal than it is today; syphilis was a major killer in Europe during the Renaissance. The first cholera pandemic began in Bengal, then spread across India by 1820. Ten thousand British troops and countless Indians died during this pandemic. Between 1736 and 1834 only some 10% of East India Company's officers survived to take the final voyage home. Waldemar Haffkine, who mainly worked in India, who developed and used vaccines against cholera and bubonic plague in the 1890s, is considered the first microbiologist.