Complex sentence: But the Yahgan, who never numbered more than 3,000 individuals, could not survive contact with white man's diseases; they allegedly became sick immediately if the missionaries persuaded them to put on some clothes.
Split into simpler sentences: But the Yahgan, who never numbered more than 3,000 individuals, could not survive contact with diseases brought by Westerners. They allegedly became sick immediately if the missionaries persuaded them to put on some clothes.