Whether nutritional control can retard senescence of immune function and decrease mortality from infectious diseases has not yet been established; the difficulty of establishing a model has made this a challenging topic to investigate. Caenorhabditis elegans has been extensively used as an experimental system for biological studies. Particularly for aging studies, the worm has the advantage of a short and reproducible life span. The organism has also been recognized as an alternative to mammalian models of infection with bacterial pathogens in this decade. Hence we have studied whether the worms could be a model host in the fields of immunosenescence and immunonutrition. Feeding nematodes lactic acid bacteria (LAB) resulted in increases in average life span of the nematodes compared to those fed Escherichia coli strain OP50, a standard food bacteria. The 7-day-old nematodes fed LAN from age 3 days were clearly endurable to subsequent salmonella infection compared with nematodes fed OP50 before the salmonella infection. The worm could be a unique model to study effects of food factors on longevity and host defense, so-called immunonutrition. Then we attempted to establish an immunosenescence model using C. elegans. We focused on the effects of worm age on the Legionella infection and the prevention by immunonutrition. No significant differences in survival were seen between 3-day-old worms fed OP50 and 3-day-old worms infected with virulent Legionella strains. However, when the worms were infected from 7.5 days after hatching, the virulent Legionella strains were obviously nematocidal for the worms' immunosenescence. In contrast, nematodes fed with bifidobacteria prior to Legionella infection were resistant to Legionella. C. elegans could act as a unique alternative host for immunosenescence and resultant opportunistic infection, and immunonutrition researches.