Shaheed-e-Mehraab, Ayatullah Dastghaib Shirazi (r.a.) narrates an incident from Ayatullah Uzma Shaikh Abdul Kareem Haayeree (r.a.), the founder of the Hauz-e-Ilmiyyah in Qum. Janab Haayeree (r.a.) narrates:
I was residing in Samarrah to pursue religious studies. Once, the city was struck by a deadly plague. Scores of people were dying every day. During those days, a group of religious scholars had gathered at the house of our teacher Late Syed Mohammed Fesharki (r.a.). At that time, Janab Mirza Mohammed Taqi (r.a.) too came in that meeting. The talk turned towards the topic of plague and the devastation it was causing in terms of loss of lives. Marhoom Mirza Taqi (r.a.) said, “If I command you to perform an action will all of you obey me?” Everyone replied “Yes, of course, we will obey whatever you say.” Marhoom Mirza (r.a.) then ordered “Tell every Shia of Samarrah to recite Ziyarat-e-Ashura for the next ten days and to gift the reward of the Ziyarat to the soul of the mother of Imam-e-Zamana (a.t.f.s.), Janabe Nargis Khatoon (s.a) so that the people remain safe from this disease.”
All those who were present in that gathering passed this information to the Shias of Samarrah, who all began reciting Ziyaarate Aashoora. From the very next day, there was no news of any Shia death. Only the news of the death of the Ahle Sunnat was being reported. Gradually the Sunnis realized this and asked some of their Shia friends as to the reason that why none from the Shias were dying of plague. The Shias said that they were reciting Ziyarate Ashura. On hearing this, the Sunnis too began reciting the Ziyarat. Consequently, they too were secured from the perilous effects of the plague.
Source: Ziyarat-e-Ashura – Effects & Miracles, pg. 24