In species from three widely divergent phyla (Arthropoda, Mollusca and Chordata) tyrosine derivatives (norepinephrine or octopamine) mediate a response to acute stress. Part of this response is a change in immune function that results in a decrease in resistance to pathogens. This decrease in disease resistance appears maladaptive. However, if the connections between norepinephrine/octopamine and immune function were maladaptive, they should have been selected against. None of the four commonly proposed adaptive explanations for acute stress-induced changes in immune function fit the available data for species from all three phyla. However, this result is probably due to the lack of information about acute stress-induced immunosuppression in invertebrates and a lack of ecologically valid studies in vertebrates. Understanding why immune function and disease resistance changes during acute stress will require greater comparative study.