Week deviates from the classical course involving a phase in which bone marrow is infected. In this alternative course, cats exposed to low loads of virus via faecal route seroconverted without involvement of bone marrow and without showing any other sign of infection [8] . [16] . In this study, we demonstrated that a single oronasal exposure to FeLV as low as 10 000 FFU is sufficient to elicit an antibody response and to lead to seroconversion in at least 18% (lower 95% confidence interval of the distribution) of the cats, demonstrating that the postulated alternative course of infection may occur in a considerable portion of the animals. We propose to designate this outcome of infection as abortive infection with seroconversion. No hallmark of infection was observed in the cats that received the smallest of the three doses (1 000 FFU), for which individual local innate immunity might have been enough to contain viral replication and to lead to abortive infection without a detectable adaptive immune response. However, under field conditions, multiple exposures in the range of 1 000 FFU or lower may occur giving rise to seroconversion or even provirus positivity.