Fact 1: Microorganisms can cause infections.

Fact 2: Leeches clean away the dead or infected tissue.

Given the two facts above, answer the question "Dead tissue can be caused by what?" with the following options: 
- disease
 - pollen counts
 - advanced mathematics
 - microorganisms
 - migrating birds
 - viruses
 - Physical weathering
 - harmful substances


 

    microorganisms     