Write a fact related to the given fact, based on the given topic word. Note that, your fact should have at least one word in common with the given fact. All facts in this task refer to scientific facts. Your related fact must form a chain with the given fact. Chains form when two facts connect together to produce the third fact. An example of a chain is: "pesticides cause pollution" (given fact) + "pollution can harm animals" (related fact) → "pesticides can harm animals" (connected chain). Avoid creating simple paraphrases of the given fact. While your generated fact should be related to the input fact, they must describe slightly different scientific phenomena. It's okay if your related fact includes some irrelevant information, provided that it has some overlap with the given fact and it contains some words corresponding to the provided topic.

Fact: Active immunity results when an immune response to a pathogen produces memory cells. 
Topic: immune system keeps people from getting sick.
People with weaker immune systems are more likely to get sick from contaminated foods.

Fact: pasteurization reduces the amount of bacteria in milk. 
Topic: bacteria found in milk.
Raw milk has occasionally been found to carry the diphtheria bacteria.

Fact: Fish have a centralized nervous system with a brain. 
Topic: fish brain.

Eventually the chondral skull covers the brain dorsally in bony fishes.