Many of us here think they DO cause weight gain. In the last 7 months of intensive deworming, my wife went from 210# to 145#. Initially, we had *no clue* we were infected, except the vague feeling of getting older too fast. Much of the weight she lost was in roundworms themselves (they came out in droves), as well as various 'nest' debris, candida, mucous, etc. Roundworms will make your gut area swell, as they multiply.
Tapeworms are known to cause one to lose weight, and some genius doctors are even getting on the 'give people tapeworm' bandwagon. Either kind will get first shot at the ever so expensive food you eat. If you have parasites, they are taking up weight and space in your body. There has been some speculation about them influencing dietary choices and causing cravings. They apparently love simple starches, Sugar and fat, and IMO, will make one reach for that stuff. Also, one may be constantly hungry, as the nutrients in the food are first nabbed by them.
I see overweight people out in public which I immediately attribute to worm infestations EVERY DAY. And they have no idea.. Not being naive anymore, I will definitely agree that most if not all health issues can be attributed to rampant parasite infections, including 'old age'.