Social media is being increasingly used as a communication channel. Among social media, web forums, where people in online communities disseminate and receive information by interaction, provide a good environment to examine information diffusion. In this research, we aim to understand the mechanisms and properties of the information diffusion in the web forum. For that, we model topic-level information diffusion in web forums using the baseline epidemic model, the SIR(Susceptible, Infective, and Recovered) model, frequently used in previous research to analyze disease outbreaks and knowledge diffusion. In addition, we propose an event-driven SIR model that reflects the event effect on information diffusion in the web forum. The proposed model incorporates the effect of news postings on the web forum. We evaluate two models using a large longitudinal dataset from the web forum of a major company. The event-SIR model outperforms the SIR model in fitting on major spikey topics that have peaks of author participation.