instruction:
Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
question:
Fact 1: gills are used for breathing water by aquatic animals. 
Fact 2: Fish are backboned aquatic animals.
answer:
fish use gills for breathing water.


question:
Fact 1: Some parasites kill their host, but most do not. 
Fact 2: Tapeworms are internal parasites of vertebrates.
answer:
most tapeworms do not kill their host.


question:
Fact 1: Active immunity results when an immune response to a pathogen produces memory cells. 
Fact 2: If the parasite causes harm, it is a pathogen.
answer:

When a parasite produces memory cells, active immunity results.