strange behavior when using linux shell

cat testfile

aa
bb
cc

#!/bin/sh

ruleID=100

cat testfile | while read line
do
case $line in
aa)
echo "@@@aa@@@ruleID = $ruleID"
ruleID=10
echo "@@@aa@@@ruleID = $ruleID"
;;
bb)
ruleID=20
echo "@@@bb@@@ruleID = $ruleID"
test $ruleID
;;
esac
echo "@@@out of case@@@ruleID = $ruleID"
done

echo "@@@out of do while@@@ruleID = $ruleID"

sh aa.sh
@@@out of case@@@ruleID = 100  //the ruleID's value get out of do while loop
@@@aa@@@ruleID = 100
@@@aa@@@ruleID = 10
@@@out of case@@@ruleID = 10
@@@bb@@@ruleID = 20
@@@out of case@@@ruleID = 20
@@@out of case@@@ruleID = 20
@@@out of do while@@@ruleID = 100   //the ruleID's value come back!!