One quarter of the world population is infected with parasitic worms that are called helminths. To avoid being eliminated these parasites actively dampen the immune response of their hosts. This helminth-induced immune modulation also suppresses immune responses to third-party antigens. As a consequence, helminth-infected people may not mount successful immune responses to concomitant infections or to vaccinations. We use the parasitic nematode Strongyloides ratti to investigate this in the mouse system. We are interested in (i) the mechanisms of protective anti-helminth immunity, (ii) the mechanisms of helminth-induced immunomodulation and (iii) the impact of concurrent helminth infection on the outcome of a vaccination against different pathogens.