Joseph, in a letter to the Quorum of the Twelve Apostles, cautions them from returning from Britain during "sickly season."

Date: Dec 15, 1840
Type: Letter
Source: Joseph Smith, Jr.
LDS
Hearsay: Scribed Verbatim
Direct
Reference: Joseph Smith, Letter to the Quorum of the Twelve Apostles, December 15, 1840, The Joseph Smith Papers website, accessed January 3, 2024
Scribe/Publisher: Robert B. Thompson
People: Joseph Smith, Jr., Quorum of the Twelve Apostles
Audience: Quorum of the Twelve Apostles
Transcription: With respect to the rout best to be taken I think you will be better able to <give> advise than myself. But I would not advise coming round by the lakes. And it would not be prudent to come via New Orleans in the sickly season. but in the spring or fall or winter it might do. Give my kind love to all the brethren, and sisters, and tell them I should have been pleased to have come over to England to see them, but am afraid that I shall be under the necessity of remaining here for some time, therefore I give them a pressing invitation to come and see me.
Source Link: https://www.josephsmithpapers.org/paper-summary/letter-to-quorum-of-the-twelve-15-december-1840/7
BHR Staff Commentary: Note 56 on The Joseph Smith Papers website explains "sickly season":
Along the Mississippi River, more people contracted malaria in the summer months. In the mid-nineteenth century, this increase in malaria cases was seen as a result of seasonal miasmas, or vapors caused by “the decomposition of vegetable or animal matter.” (“Westminster Medical Society,” 849–851.)
Citations in Mormonr Qnas: Prophetic Fallibility