As a part of a research-and-action partnership between public health and veterinary medicine, the relationships between the seroprevalences of brucellosis and Q-fever in humans and livestock were evaluated in three nomadic communities of Chad (Fulani cattle breeders, and Arab camel and cattle breeders). Nomad camps were visited between April 1999 and April 2000. A total of 860 human and 1637 animal sera were tested for antibodies against Brucella spp., and 368 human and 613 animal sera for Coxiella burnetii. The same indirect ELISA was used for livestock and human sera, and the test characteristics for its use on human sera were evaluated. Twenty-eight people were seropositive for brucellosis (seroprevalence 3.8%). Brucella seroprevalence was higher in cattle (7%) than other livestock, and brucellosis seropositivity was a significant factor for abortion in cattle (OR=2.8). No correlation was found between human brucellosis serostatus and camp proportions of seropositive animals. Q-fever-seropositive blood samples were taken from 11 Arab camel and 4 Arab cattle breeders (seroprevalence 1%). Being a camel breeder was associated with Q-fever seropositivity in humans (OR=9). Camels had the highest Q-fever seroprevalence (80%) among livestock species. Although there was high-risk human behaviour for the acquisition of brucellosis and Q-fever from livestock through raw-milk consumption (98%) and contact with placentas of livestock (62%), we concluded that seroprevalences in humans were relatively low (likely due to limited active foci in livestock).