Hi

A lot of people with proven infections have multiple parasites. For example studies mention that children in Africa often have at least 3-4 different types of parasites.

One of the problems noted with helminths is that a lot of them modulate the immune system to their advantage -this means that the host's immune system does not mount the usual defence, it does not produce the usual inflammation and "enemy attack" by the cells and substances that help us kill off bacteria, viruses and any parasites.

It is easy to see how that would make someone vulnerable to multiple parasites, once their immune system is switched off.
(This is a principle that researchers are trying to use in order to control illnesses that are auto-immune or due to excessive inflammation like asthma, Crohn's disease, etc. I personally think this is a slippery slope - they infect someone with something they cannot be entirely sure they can eradicate, given that the vast majority of these parasites are so hard to detect!)

Something else that can happen is that bacteria e.g. from the gut or the respiratory tract can enter through the holes left behind by the parasites and thus cause infections - this can further weaken the body.

Another reason for multiple parasites might be that if you contract an infection from e.g. an animal that has not been dewormed - that animal itself is likely to have multiple parasites and may transmit several at the same time.

Parasites are unfortunately very good at harnessing the host's immune system & modulating it in order to ensure long-term survival without killing off the host.