humans are constant host to a stream of bacteria, and it is by mere chance that some of the bacteria have waste that is toxic to humans.

여기서 have waste that is toxic to humans. 저 that이 앞에 있는 bacteria 이고, 다음에 (that) 이 생략되서have waste that (that) is toxic to humans. 라고 봐서 해석해 줘야 되는건가요?