Decades ago researchers first began to find clues indicating that there is in fact an upside to parasites. In 1968, a Brian M. Greenwood, of the London school of hygiene & tropical medicine, found that people of Nigeria had lower levels of both arthritis and other immune-diseases then people living in Britain. He deducted that the chronic infection of intestinal worms or other parasites moderated the immune system in Nigerians, this then reduced the risk of their bodies own tissues attacking themselves.
Evidence then came over the decades and ended up known as the Hygiene hypothesis. Supporters of this hypothesis have argued that over the centuries, humans in modern civilization has radically changed our relationship with parasites. Evolving to have an immune system to fend off infections also needs to tolerate mild attacks on the system or it would lead to destroying the beneficial bacteria, or even damage its own tissues with inflammation. So our developing immune system became to depend on parasites to help the immune system develop properly. However that is not saying that parasites are all good, our relationship with them has instead become a complicated trade-off, constantly being reassessed by our bodies.
With humans it was not until the 19th century when the relationship with parasites changed, thanks to clean food and water being made available to the populations, this twined with the Industrial Revolution that took people out of the fields, meant less contact with soil and microbes. With this happening to the people,it is no coincidence that people in industrialized countries have experienced more allergies, asthma, crohn's disease as well as other immune-related disorders. For example having worms when you are younger can lead to fewer allergies when older, but you could possible stunt your growth if their population becomes uncontrollable, so its a fine line between benefit and hindrance. The Hygiene hypothesis is now a known and proven hypothesis within humans,the argument for some conservationist is, can it affect captive animals as well.
In the wild, a animal within a healthy environment may have a multitude of of intestinal worms and other inhabitants. animals that are kept in captivity and in breeding populations, will often be treated for infections and parasite treatment, that can often be using aggressive medication. Whist these medications can save the animals lives, and can improve their quality of life as well, it may have a downside. The use of aggressive medicine will remove parasites that can aid the animals immune systems as well as helpful gut bacteria, with these parasites & bacteria removed the animals may become unprepared for diseases that they may encounter in the wild.
With most parasites they specialize on one species or a group of species, with their food group being removed from the environment this can break the link between the two organisms. Parasite extinction isn't just a theoretical problem, when California biologists brought all the surviving California condors into captivity in 1980's, they ended up eradicating the Colpocephalum californici, a type of feather louse only found on these birds. So by 'saving' one species, you are taking the fate of their parasites in your hands, and dooming many of them to pages in history books. We can be wiping out other parasites without even realizing it, by de-worming all endangered Kakapos that remain in captivity, humanity could have just driven a species of tapeworm to extinction.
It is possible that the lack of parasites that are found in reintroduced animals could be the reason behind the disappointment in many rewilding figures. By sending out animals with no parasites or immune system to tackle the world in now finds itself in, we are not sending them out with the best possible chance. However the plight of the parasite will be a hard sell, with many arguing the quality of life of the animals that they inhabit and the destruction that they can cause.
But maybe, by not treating the hell out of everything before sending the animals back out in the wild, allows for rewilding to occur before changing their enviroment, and possibly be saving a few other species along the way,