Pertinacious comes from French pertinacité (early 15c.), from Old French pertinace "obstinate, stubborn," from Latin pertinacem (nominative pertinax) "very firm, tenacious, steadfast, persevering," from per "very" (see per) + tenax (see tenacious).  Pertinacity is a quality of sticking with something, no matter what.  It's a type of persistent determination. 

The first known use of the word "pertinacious" in English was in 1583. It was borrowed from Latin, with the meaning of being firmly attached, tenacious, and obstinately persistent. 

The word "pertinacity" first appeared in English around 1495-1505.  The OED's earliest evidence for pertinacity is from around 1504, in a translation by William Atkinson, translator. 

Examples of "pertinacious" and "pertinacity" used in a sentence: 
"He knew she was only so pertinacious out of her love for him."
"John  worked with a pertinacious resistance to interruptions and never let anything cause him to stop his work."
"Apart from his pertinacious fight for economy Hume was not always fortunate in his political activity."
"The Cabetes, who possessed the soil, were fierce and pertinacious; and, assisted by the French, who traded to that coast, Coelho had to gain by inches what was granted him by leagues." 
"She would offer them to people with her politely pertinacious manner and upper-middle-class accent and invariably had good sales."

"Her pertinacity in pursuing her degree, despite countless obstacles, ultimately led to her success." 
"Today, these artists have found recognition on a national level and their pertinacity has brought them strong following across the nation." 
"His pertinacity has been bluntly denominated stubbornness."  
"His pertinacity, saw him not give up." 
"People who have pertinacity won't give up, and they stick with things doggedly."
"Pursuing a difficult career requires pertinacity."

1 hour ago, drpaladin said:

The phrase which comes to mind is pernicious anemia.

5 minutes ago, drpaladin said:

It's a disease caused by B12 deficiency.

I wasn't commenting on the cause of the diseases, just noting the difference between the words "pernicious" and "pertinacious." 

Just the facts:

PERTINACIOUS Definition & Meaning - Merriam-Webster