In 1988, he became more visible to the general public with the dramatic death of seals in the North Sea. 7 Osterhaus was called in to investigate these deaths, and discovered that the seals had died of a previously unknown virus. However, Greenpeace opposed his conclusion: they were convinced that pollution or poisonous algae had caused the fatalities, since this fitted their own objectives. This disagreement with the environmentalists challenged Osterhaus to prove he was right, and he conducted extensive experiments with seals. To his surprise these studies did identify pollution as a contributing factor in the seals' deaths, but as an indirect causal factor: it made them less resistant. Osterhaus considered this a success: he had discredited his own hypothesis and discovered a causal chain more nuanced than either he himself or Greenpeace had expected (Osterhaus 6 March 2008). The research on seal death, that started in 1985 and has continued ever since, has had an enormous impact on several aspects of Osterhaus' career. It delivered him his first four Nature publications in 1988 and 1989, and many other papers followed. His reputation grew, and the conflict with Greenpeace pulled him into political and public debates. Due to the seal death research he also became scientific advisor to a national seal shelter.