Complex sentence: In any case, he was careless not only of his own safety, but also of the safety of those around him, and despite repeated promises to Young, failed to keep infected mosquitoes in the specially designed secure housing.
Split into simpler sentences: In any case, he was careless not only of his own safety, but also of the safety of those around him. Despite repeated promises to Young, failed to keep infected mosquitoes in the specially designed secure housing.