Zika virus provides an example for which public health surveillance is based primarily on health care provider notifications to health departments of potential cases. This case-based surveillance is commonly used to understand the spread of disease in a population. However, case-based surveillance is often biased-whether testing is done and which tests are used and the accuracy of the results depend on a variety of factors including test availability, patient demand, perceptions of transmission, and patient and provider awareness, leading to surveillance artifacts that can provide misleading information on the spread of a disease in a population and have significant public health practice implications. To better understand this challenge, we first summarize the process that health departments use to generate surveillance reports, then describe factors influencing testing and reporting patterns at the patient, provider, and contextual level. We then describe public health activities, including active surveillance, that influence both patient and provider behavior as well as surveillance reports, and conclude with a discussion about the interpretation of surveillance data and approaches that could improve the validity of surveillance reports.