When I looked your code, I found a small issue.
if your calling this method onchange of the dropdown then it wont work because the selected index will be the current index selected. So i have added a code snippet below which takes the selected index on pageload itself. May be you could understand if you take a look at it. If you face any problem with this reply me.
function DiseaseChange(id) {
var ddlObj = document.getElementById(id);
var index = ddlObj.selectedIndex;
if (confirm("Are You sure yow want to change disease?") == true) {
}
else {
ddlObj.selectedIndex = PREVIOUS_INDEX;
}
}
var PREVIOUS_INDEX = 0;
$(document).ready(function () {
var ddlObj = document.getElementById(id);
PREVIOUS_INDEX = ddlObj.selectedIndex;
});