In all reality I find it almost impossible for anybody to have that many parasites and still be alive lol not saying your lieing or anything, but seriously thats ALOT of different types of parasites.. Roundworms are said to make you lose wieght or make it so you cant gain wieght at all. Ascaris alone causes fat malabsorption!!! So you have this worm most likely you would lose wieght. Hookworm also causes wieght loss, with protein and iron defiency.. there are a bunch of roundworms that make you lose wieght. Now onto tapeworms, these lil buggers make you lose wieght almost 90% of the time, from all the research I have done and talked to people who tested positive for it, all lost wieght.
For myself I know that I have either tapeworm or roundworm and I went from 190 pounds to about 165-170..
I dont know where you got the info that people cant gain wieght with parasites, its def possible everybody is different and some people have a few worms and some people are infested...but if somebody is heavily infested, they will most likely lose wieght.