Infections with gastrointestinal nematodes are some of the most prevalent infections in the world today. Despite the fact that antihelminthic drugs are generally regarded as safe, efficient and are widely available, the level of infection remains as high or, in some cases, even higher than 50 years ago. It is clear that immune-mediated resistance to infection in mouse models is mediated by a T-helper 2-type immune response. Gastrointestinal nematodes are powerful modulators of the immune response, and as such, they may have the potential to affect the outcome of concurrent infections with other parasites, bacteria or viruses. Importantly, they also appear to have the ability to influence the development of allergic reactions and modulate the effects of vaccination.