Haemophilus parasuis is still one of the main causes of nursery mortality in most U.S. herds1. Mortality rates due to Haemophilus parasuis can be as high as 10%,2 which makes this agent one of the most costly pathogens in swine production. Although a few herds experience nursery mortality solely due to H. parasuis, disease caused by this agent may occur at the same time as other bacterial and viral infections. Streptococcus suis and Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) are two of the agents that are frequently isolated from pigs showing polyserositis due to H. parasuis.3 The epidemiology of Streptococcus suis infection is very similar to H. parasuis, which may explain why these two agents often affect nursery pigs at 4 to 6 weeks after weaning.4 PRRSV infection, however, has a very distinct role in H. parasuis infection. A recent study suggests that pigs colonized by a virulent H. parasuis strain are susceptible to development of systemic infection by this agent following PRRSV challenge.5 Although the interaction between PRRSV and H. parasuis is now evident both experimentally and in the field, the mechanisms involved in such interaction are still unclear. The present article will discuss some of the features regarding diagnosis, epidemiology, and control of H. parasuis in the nursery.