WHY SOME GET SICK

This paper reports factors that determine the susceptibility of indivi duals to bacterial, viral, and parasitic diseases and toxins, as well as their humoral and cell-mediated immune responses. Interactions of g enotypes and environments are major factors in the allocation of resou rces to disease defense. These allocations are dynamic and vary throug hout the life of an individual. Allocation of resources to defend agai nst disease may occur at the expense of growth, feed conversion, or ot her factors. Furthermore, an individual cannot simultaneously have a s trong defense against all disease agents. Virulence of infectious agen ts and host-pathogen relationships are also important factors in an in dividual's susceptibility. Since any individual's level of defense aga inst a given agent is not constant, chance is also a factor, for expos ure to a disease agent can occur when the individual's defense level i s low or high.