A man died from an infection caused by rickettsiae. The infection was most likely transmitted by:

Rickettsial infections are typically transmitted to humans through the bites of infected arthropods, primarily ticks. Certain species of ticks are well-known vectors for rickettsiae, which are a type of bacteria that can cause various diseases, such as Rocky Mountain spotted fever and typhus.

In this case, the method of transmission by a tick bite is well established in medical literature, as rickettsial diseases are endemic in certain regions and are often associated with outdoor activities where exposure to ticks is likely. The life cycle of rickettsiae involves these vectors, making ticks the most likely source for such infections in humans.

When considering other modes of transmission, walking barefoot in a locker room or breathing contaminated air does not relate to rickettsial infection spread. Blood transfusion is not a common transmission route for these types of infections either, as rickettsiae are generally not present in the bloodstream to the same extent that viruses or other bacteria might be. Thus, the definitive association of tick bites with rickettsial disease makes this option the correct choice.