How can parasites live in the intestines for years without causing any symptoms?

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How can parasites live in the intestines for years without causing any symptoms?

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For the parasite to live, they need you to live. So it’s in their best interest to not cause problems for you.

Many symptoms of disease come from your own body’s attempts to kill the pathogen. Parasites often produce immunosuppressive chemicals to hide from your immune system and weaken attacks against the parasite. Also, since they’re living inside of you is not very advantageous to kill you. They have generally evolved not to be disruptive unless they need to be.

And they usually do still cause symptoms, just often more subtle like weight loss or fatigue that may not be extreme or immediately apparent.

The parasites that are more likely to straight up kill you are the ones that evolved for a different host and ended up inside a human. Or, like malaria, they just gtfo before you perish from the damage they caused.

Additional info: Depending on where a specific parasite is in its life cycle it may not cause symptoms. Tapeworms for example generally don’t cause immediately noticable symptoms until they’ve reached maturity because if they activate your body’s immune system, they create risk for themselves and the host they feed on. When, however, it’s time to perform the tapeworm tango, they stop caring about the host conditions and are more concerned with ahem *exiting* the body for breeding.