A monogamous partnership and biparental care, which occur in various vertebrates and invertebrates, pose both advantages and risks, including mutual disease transmission. In this situation, the immune strength of one’s partner should greatly influence one’s own survivorship. Male and female primary reproductives　cooperatively establish a new colony in termites, so colony foundation success depends on the survival of both primaries. Here we show that colony foundation success by a pair is highly predictable from individual-level resistance by a “drag-down” model, that is, the individual of lower-resistance determines the fate of new colony rather than the higher-resistance partner. We found a significant colony-dependent sex difference in personal immunity when primaries of the termite Reticulitermes speratus were kept isolated after exposure to the pathogenic fungus Metarhizium anisopliae. By linking the data from the isolation experiment and the pairing experiment, we found that because the partner left alone eventually dies, the one having a lower resistance level determines whether the colony will be founded successfully.