He helped popularize the idea that some diseases not previously thought to have a bacterial cause were actually infections, which aroused much controversy when it was first suggested.

过去分词短语作定语，不是很常见的吗？只不过是否定而已。not thought to have..（之前未曾被认为有...）

又如：

Similar phenomena are reported by mountain climbers, a group generally not thought to be very mystical.
一群普遍认为不很神秘的登山者报告了类似的现象。