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.

Fact 1: Viruses cause many human diseases. 
Fact 2: Human viruses are created by natural process.
Something created by a natural process  cause many human diseases.

Fact 1: Gene flow occurs when individuals move into or out of a population. 
Fact 2: Geographic subdivision decreases gene flow.
Geographic subdivision decreases how fast individuals move into or out of a population.

Fact 1: faulting of rock in Earth 's crust causes earthquakes. 
Fact 2: Earthquakes generate P waves and S waves which can travel throughout the earth.

P-waves and S-waves can be traced back to faulting of rock in Earth's crust.