decreasing something negative has a positive impact on a thing & illness has a negative impact on an organism 's health ->  decreasing illness has a positive impact on an organism's health  decreasing illness has a positive impact on an organism's health & preventing something decreases the amount of that something ->  preventing illness has a positive impact on an organism's health  preventing illness has a positive impact on an organism's health & vaccines can help prevent illness ->  vaccines have a positive impact on an organism's health by helping to prevent illness  vaccines have a positive impact on an organism's health by helping to prevent illness & having a positive impact on something helps that something -> vaccines help organisms by helping to prevent illness