A few more details. Of special note is the possible danger of taking a TNF blocker.

more histoplasma => more TNF-alpha
"TNF-α is a key modulator of disease in both primary and secondary histoplasmosis" [1].

more TNF-alpha => more damage to the substantia niagra
"Chronic expression of low levels of tumor necrosis factor-alpha in the substantia nigra elicits progressive neurodegeneration" [2]

more damage to the substantia nigra => more progression of PD
"Parkinson's disease is a neurodegenerative disease characterized, in part, by the death of dopaminergic neurons in the pars compacta of the substantia nigra." [3]

more progression of PD => more TNF-alpha
"TNF and solTNFR1 levels are elevated in cerebrospinal fluid and tissues of PD patients as well as in postmortem PD brains with the highest TNF levels present in areas that have the greatest loss of dopaminergic neurons" [4]

more TNF-alpha => better defence to histoplasmosis
"FDA ALERT [9/4/2008]: FDA is notifying healthcare professionals that histoplasmosis and other invasive fungal infections are not consistently recognized in patients taking tumor necrosis factor-α blockers (TNF blockers), Cimzia (certolizumab pegol), Enbrel (etanercept), Humira (adalimumab), and Remicade (infliximab). This has resulted in delays in appropriate treatment, sometimes resulting in death." [5].

References:

[1] "Histoplasma Virulence and Host Responses"
Mircea Radu Mihu and Joshua Daniel Nosanchuk
International Journal of MicrobiologyVolume 2012 (2012), Article ID 268123, 5
pagesdoi:10.1155/2012/268123
http://www.hindawi.com/journals/ijmb/2012/268123/

[2] "Chronic expression of low levels of tumor necrosis factor-alpha in the substantia nigra elicits progressive neurodegeneration, delayed motor symptoms and microglia/macrophage activation."
De Lella Ezcurra AL, Chertoff M, Ferrari C, Graciarena M, Pitossi F.
www.ncbi.nlm.nih.gov/pubmed/19969084

[3] http://en.wikipedia.org/wiki/Substantia_nigra

[4] "TNF signaling inhibition in the CNS: implications for normal brain function and neurodegenerative disease"
Melissa K McCoy and Malú G Tansey
Journal of Neuroinflammation 2008, 5:45
http://www.jneuroinflammation.com/content/5/1/45

[5]
http://www.fda.gov/drugs/drugsafety/.../ucm124185.htm

John