An outbreak among homeless shelter users of a communicable disease with a short generation time would pose serious public health challenges. Data from Toronto were used to examine the number of shelter residents potentially exposed in the event of such an outbreak. A shelter user had contact with a mean of 97 other residents (range, 1–292) in one day and a mean of 120 (range, 2–624) in eight days. After a single week, contact tracing becomes difficult due to the challenge of locating homeless people who have left the shelter system. Over an 8-day period, individuals who used more than one shelter had contact with an average of 98 more other shelter residents than those who stayed in a single shelter had. At the onset of a serious outbreak, it may be desirable to institute policies that strongly encourage individuals to remain at their current shelter for the duration of the outbreak.