PROS1 and viral infectious disease: Stroke due to infectious arteritis accounted for 9% of the causes of ischaemic stroke in the study by Mbaye et al.11 Viral infections, particularly upper respiratory infections, and varicella, as found in our study, are described as the most frequent causes of transient focal arteriopathy.5 These infections are thought to lead to a state of hypercoagulability due to a transient drop in the free fraction of protein S, particularly in the case of viral infections.23,24