Wildlife populations that are under pressure from a variety of threats are in consequence also at an increased risk of disease.
Disease as a threat in its own right is often only taken into account after some serious outbreak such as chlamydia in koalas or the facial tumour in Tasmanian devils. 

The major threats – climate change, habitat loss, presence of predators – can incite a physiological stress response and lower immune function. Stress can also exacerbate impacts of diseases and further increase a vulnerable species’ risk of extinction.
The association between stress and disease in wildlife are not commonly empirically tested and it is unknown to which extent for instance the emergence of infectious disease in wildlife is influenced by stress . (Hing et al, 2016)

Stress in connection with capture, handling, radio-collaring etc. is an unavoidable issue when translocating animals but it has been suggested that this stress could directly influence translocation success or failure. Latent pathogens could lead to disease in an immunosuppressed host.

There might also be pathogens at the release site to which the animals have not been exposed before which could render them unable to establish or the translocated animals might carry pathogens that are novel to the resident population on site so that they cannot produce an effective immune system response.  (Hing et al, 2016, Ewen et al, 2015)

Animals that are sourced from an unregulated care situation and are released without a health check could seriously endanger an extant population.

Judy Clarke’s PhD research (2011) is the only (available) study so far that looks into the role of health/disease in ringtail possum survival after translocation.  However, she only looked at a small range of severe diseases that also have zoonotic potential. Those were obviously not the key disease hazards for western ringtail possums. Their probabilities of occurrence were quite small but the impact if they occurred would have been major. 

She acknowledged that some organisms are capable of causing disease in stressed and immunosuppressed hosts, but her findings were generally negative regarding the likelihood of activation of latent disease by the stress associated with relocation of possums. (Clarke et al, 2013)
A few ringtails that died in bad body condition and with high endoparasite burdens but not displaying any signs of predation could represent stress victims.  Animals were also killed so quickly after release that it is possible that there were other underlying issues that made them more vulnerable to predation.

Even though the tested diseases had no effect on ringtail survival, pre-release white blood cell counts turned out to be one of the two factors heavily influencing post-translocation survival and this could still be hinting at low level infection or compromised health as a major issue. (Clarke, 2011)

However, differences in haematology values at different sites (before and after translocation) showed the importance of habitat quality and nutrient intake. The nutritional quality of vegetation at the translocation sites seemed lower than in the source regions where ringtails are abundant. The lower haematological values post-translocation could have been due to low foliage nutrient levels, high toxic secondary metabolite concentrations that limit food intake and/or limited access to food due to competition – or all of the above. (Clarke, 2011)

Feral cats are usually implemented in the spread of disease in marsupial translocations as they carry a wide range of parasitic and disease-causing organisms.  Cats are the definitive host for Spirometra erinacei, a pseudophyllidean tapeworm (Dickman, 1996, Reddacliff and Spielman, 1990) and toxoplasma gondii, a protistan parasite that can form cysts in the nervous and muscular tissue of avian and mammalian hosts. (Dickman, 2014)
Both can cause severe clinical symptoms in native wildlife.

An infection by T gondii can lead to respiratory and enteric problems, ataxia, lethargy and eventual death. (Thompson et al, 2010)
However, according to the literature covering western ringtail translocations only 2 cases were definitely diagnosed in about 25 years. The risk might be higher in urban populations with exposure to domestic cats, but I am not aware of a single proven case.

Foxes can potentially also affect native fauna through the spread of disease, such as round-worm, scabies and hydatid tapeworm (Saunders et al, 2010) but this is rarely mentioned or investigated.

There are now indications that a potential disease with clear conservation significance for western ringtail possums is related to their food source.
Translocations to sites with lower food quality or tree decline and changes to tree quality in the core area through long drought periods could mean extended times on suboptimal nutrition with high toxicity paired with low water availability. Critical impairment of the digestive system and urinary tract could be the consequence.
Ringtail mortality is usually high during long drought periods but we also experienced a number of ringtails with calcium based bladder stones that would lead to a certain death in the wild.

Leaves on Leschenault Peninsula showed particularly high calcium oxalate levels but as only leaves from a small range of habitats were tested, it is unknown whether this is not also the case in other areas and/or a general feature of older, drier foliage.