That's part of it, but intestinal parasites also interfere with normal digestive processes. Their presence and secretions (waste products and the compounds they use to prevent being digested) can cause a lot of problems. It's common for people with intestinal parasites to have reduced appetite, intestinal inflammation, and digestion issues.
The weight loss isn't just a result of the parasite competing for nutrients, though that doesn't help.
The weight loss isn't just a result of the parasite competing for nutrients, though that doesn't help.