Many current mobile health applications ("apps") and most previous research have been directed at management of chronic illnesses. However, little is known about patient preferences and design considerations for apps intended to help in a post-acute setting. Our team is developing an mHealth platform to engage patients in wound tracking to identify and manage surgical site infections (SSI) after hospital discharge. Post-discharge SSIs are a major source of morbidity and expense, and occur at a critical care transition when patients are physically and emotionally stressed. Through interviews with surgical patients who experienced SSI, we derived design considerations for such a post-acute care app. Key design qualities include: meeting basic accessibility, usability and security needs; encouraging patient-centeredness; facilitating better, more predictable communication; and supporting personalized management by providers. We illustrate our application of these guiding design considerations and propose a new framework for mHealth design based on illness duration and intensity.