Individual differences in behaviour are a phenomenon that is more and more attracting the attention of scientists. Among the other reasons, behavioural individuality occurs because selection favours the adoption of different tactics by individuals. It is now widely recognized that within many vertebrate species, individuals vary along an axis the extremes of which are represented by individuals 'bold' and 'shy', sometimes called 'proactive' and 'reactive'. Here we present the case of feral domestic cats (Felis catus L.) living in group in the urban environment where showing bold attitudes is linked to the benefit of a high annual reproductive success but, on the other hand, to a high probability to be infected by the Feline Immunodeficiency Virus (FIV), a lethal disease caused by a retrovirus. In this species, natural selection has probably favoured proactive temperament in spite of the cost represented by getting the disease. In fact, proactive individuals, even if FIV positive, reproduce more than reactive individuals before the last stage of FIV-infection (AIDS) characterized by a loss of immunological defences and subsequent opportunistic infections. Evolutionary implications are discussed.