Dr. Derval Igoe:

Nobody knows for sure. What possibly happened was that a person had a link to an endemic country and then, within a smallish population where there are quite a lot of links, a seeding event or several seeding events happened whereby that person was in touch with other people within that community who then became infected. There were probably what we would call chains of transmission that were undetected at that time. The second thing that was unusual in May of this year was that the clinical manifestation and severity of the disease was milder than what would have been recognised in Africa.