Since Retroviral Hyperplasia isn't a real disease I'm going to assume that Retroviral Hyperplasia is an Endogenous retrovirus (ERV). ERVs are endogenous viral elements in the genome that closely resemble and can be derived from retroviruses. Most are ancient, inactivated by mutation, and have reached genetic fixation in the host species, and rarely cause trouble for the host. But researchers have proposed a role for newer ERVs in several forms of human cancer and autoimmune disease. Specifically, ERVs have been proposed to be involved in multiple sclerosis (MS). A specific association between MS and the ERVWE1 gene, which is derived from an ERV insertion, has been reported, along with the presence of an "MS-associated retrovirus" (MSRV), in patients with the disease. Human ERVs (HERVs) have also been implicated in ALS.
Antibodies to HERVs were found in greater frequency in the sera of people with schizophrenia. Additionally, the cerebrospinal fluid of people with recent onset schizophrenia contained levels of a retroviral marker, reverse transcriptase, four times higher than control subjects. Researchers continue to look at a possible link between HERVs and schizophrenia, with the additional possibility of a triggering infection inducing schizophrenia.
MECHANISM OF DISEASE:
Because one of the first and most pronounced symptoms Harry experiences is a persistent and gradually worsening tremor, the evidence suggests that the disease's primary mechanism is neurological. This would also partially account for the paranoia, mood swings, and other psychiatric symptoms that Harry has experienced. The serum, not an effective treatment for this ailment, will instead amplify his psych symptoms and pretty much fuck him up by further modifying his genetic code without doing anything to really help some of the more glaring problems with the existing modifications.
The mechanism of the disease combines elements of Wilson's disease, Huntington's disease, and KoRV.
Similarities to Wilson's Disease: Although the disease is genetic and can worsen over time, the psychiatric symptoms are not necessarily progressive and permanent. Though the neurodegenerative symptoms of the other diseases can yield similar results, Wilson's disease in particular is sometimes mistaken for schizophrenia, and the psychiatric symptoms often abate considerably when the underlying cause is treated effectively. (In the case of Wilson's, copper buildup.) That said, psych manifestations of Wilson's are confusing, irregular, and rarely sufficient for a diagnosis, all of which suggests some other predisposition to psychiatric symptoms in sufferers who observe them, which is true in Harry's case. On this count, note the role of ERVs in the development of schizophrenia, which can be treated by means other than antiretrovirals, and is not necessarily progressively neuro-degenerative.
Similarities to Huntington's Disease: It appears later in life, it sucks, there's not much you can do about it, it causes degenerative psych symptoms and mood disturbances as well as tremors and other neurological symptoms. But Harry's disease, though neurological, will not be irreversibly neurodegenerative.
Similarities to KoRV:Koala retrovirus, an ERV that causes immunodeficiency, and is basically Koala HIV. However, unlike HIV, KoRV can be passed on to offspring as a gene. It began as a virus, but the virus can be transmitted vertically, passed from parent to offspring as a gene. KoRV causes the deterioration of the host's immune system, leading to opportunistic infections. A virus that operated similarly could explain the strange array of symptoms Harry experiences, particularly the unusual skin lesions that appear throughout the film.
In short, Harry has a genetic cell abnormality/virus that is causing his cells to attack his own immune system and parts of his brain, especially his NDMA receptors. Probably will lead to encephalitis and seizures.
ALTERNATIVE:
Retrovirus that causes a proliferation of tumors (hyperplasia), including the abnormal skin growths. A pheochromocytoma type hyperplasia, where it's a pre-neoplastic hyperproliferation in the basal ganglia of the brain leading to altered activation of the amygdala, leading to altered behavior and a more primal brain.
Epidemiology linking autoimmune diseases and psychosis (x)
TREATMENT:
ERVs are treatable with antiretrovirals, though this like it may be less effective given that they are already incorporated into the host organism's genome. With that said, antiretrovirals have shown to be somewhat effective in managing MS. Harry is likely taking these. They may slow the replication of this virus, but ultimately cannot cure the ailment.
In this case, the antiretrovirals will do neither, since they're not really antiretrovirals. Gustav Fiers, in conjunction with Norman Osborn and Doctor Connors, have been covertly swapping out Harry's medication with a drug that was developed by Oscorp to advance the progression of symptoms in laboratory animals in order to test late-stage treatments. That's why the disease is progressing so rapidly, and that's why nothing is working.