Echinacea is a medicinal plant that has a history of use for the treatment of respiratory tract infections. More recently there has been a renewed interest in alternative therapies including phytomedicinal preparations of Echinacea. Some important issues that remain to be addressed regarding the use of Echinacea therapeutically include plant species specific alterations of immune response and the effect of different preparation techniques (water as compared to ethanol) on immune response to viral infection. Results from multiple studies in mice, in cell culture, and in clinical trials in humans have suggested that water preparations provide a stimulatory effect on innate immunity by enhancing macrophage activation, whereas ethanol extracts decrease inflammation associated cytokines/chemokines. However, the mechanisms by which different plant species enhance immunity against influenza infection and improve disease outcome remain to be clearly elucidated. In this dissertation, both water and ethanol extracts of three species of Echinacea were used to assess alterations in the immune response to influenza A/PR/8/34 virus infection. Studies focused on the impact of Echinacea treatment during influenza infection on 1) in vitro models of viral load and immune responses in the respiratory tract epithelial cells and immune cells, 2) combination treatment of water and ethanol extracts delivered at specific phases of influenza infection and the impact on "traditional" immune measures as well as a bioinformatics approach to lung gene regulation using the Database for Annotation Visualization and integrated Discovery (DAVID), and 3) immune response to influenza in obese mice treated with Echinacea water extract. The primary topic of this dissertation was the effect of Echinacea on immunity. However, there was also an emphasis on the topic of obesity and immunity. The results from many studies have shown diet-induced obesity related immune dysfunction in the context of the immune response to influenza virus and vaccination. In this dissertation, the effect of Echinacea treatment on obesity associated immune impairments to influenza infection was also evaluated.