Tchaikovsky passed away on November 6, 1893 (October 25, by the old Russian Julian calendar) from cholera, which was a serious public health concern back then and could spread rapidly if the infected water is consumed unknowingly. While this may seem "unusual" to many today, cholera outbreaks were not particularly uncommon at the time.
At that time, there was an outbreak of cholera going on around St. Petersburg (present-day Leningrad in Russia), where Tchaikovsky resided. He unfortunately contracted the bacteria, which affected his health gravely over several days ultimately contributing to his death.
To prevent speculation and controversies later on, the diagnosis from doctors stated a probable infection from contaminated water or unboiled food Tchaikovsky might have inadvertently consumed. No unusual theories exist about his demise apart from his illness leading to death.