In this paper, we analyze the content of the most popular videos posted on YouTube in the first phase of the Zika-virus outbreak in 2016, and the user responses to those videos. More specifically, we examine the extent to which informational and conspiracy theory videos differ in terms of user activity (number of comments, shares, likes and dislikes), and the sentiment and content of the user responses. Our results show that 12 out of the 35 videos in our data set focused on conspiracy theories, but no statistical differences were found in the number of user activity and sentiment between the two types of videos. The content of the user responses shows that users respond differently to sub-topics related to Zika-virus. The implications of the results for future online health promotion campaigns are discussed. © 2018 Copyright held by the owner/author(s).