whoa whoa...hold on there
first of all thers nothing that connects the re3 and re 4 as re 4 has ganados not zombies.
and if jill was as per ur poitn of view being infected even after she was cured im sure she would have turned into a zombie by end of re 3 which she dint....she was cured and thats it.
and in re 5 i dnt think the parasite was speard by jill as its due to wesker and extended company of tricell...chris along with sheva goes there to find the orgign for that parasite or whateva they call it as
jill's tombstone is still not to be considered as sign of her death as she could also be hiding just to be away from tricell or umberlla so she can investigate further and put an end to it
anyways here's the link abt jill u may find it usefull :
http://residentevil.wikia.com/wiki/Jill_Valentine