Sickness behaviours are believed to be an adaptive response to infection. However, the degree to which these behaviours can be expressed may be impacted by an individual's social environment. Here we tested, first, whether parasitism reduces the activity behaviour of lambs, Ovis aries, second, whether this occurs prior to other observed costs of parasitism and, third, whether the infection status of other in-dividuals affects the degree to which these behaviours are expressed. Sixty lambs were separated into replicate groups within three treatments: (1) parasitized: all lambs were infected with the parasitic nematode Teladorsagia circumcincta; (2) nonparasitized; all lambs were dosed with water; (3) mixed: some of the group were infected and some were dosed with water. Activity behaviour was monitored before, during and after parasite infection. Parasitized groups had reduced activity levels following infection, and this occurred before any other impact or measure of parasitism was detected. Infected animals in the mixed groups had reduced activity levels following infection, but the level of change was less than that in animals in the fully parasitized groups. Activity levels remained low until lambs were treated with anthelmintic when activity levels of the groups that had been parasitized returned to the same level as nonparasitized groups. These findings show that parasite-induced behavioural changes occur earlier than other more commonly observed signals of infection, but the infection profile of an individual's group can shape these behavioural responses to infection.