It's well known that some viruses and other parasites can influence host behaviour and physiology to maximize their spread.
Respiratory viruses make you cough and sneeze. Rabies makes you aggressive (it is spread by biting) and hydrophobic (dehydration concentrates the virus in your saliva). Toxoplasmosis replaces a mouse's instinctive fear of cats with an attraction to them (Toxo has a two-phase, mouse-cat-mouse life cycle).
A blood borne virus like HCV falls somewhere between a 'flu virus and rabies in its ease of spread. It relies mainly on injections and transfusions, being dependent on blood to blood transmission.
What adaptations and manipulations of host physiology and behaviour would promote the spread of a virus that can today only be spread effectively by licit and illicit medical procedures?
Dysphoria comes to mind. Feelings of depression and pain for no good reason; slow recovery from other insults, bad hangovers from alcohol that cannot be relieved by more alcohol. Hypochondria that exposes one to both medical procedures (means of transmission in past decades, and in 3rd world nations today) and to oral medications that can decrease ones sense of caution. Tending to lead to, and reinforce, less hygiene and more sharing among addicts. Poverty that makes blood donation a necessity (a good means of spread in past decades).
In the modern world, a blood-borne disease that makes you feel bad in every way, but leaves you well enough to find a means to ease this temporarily, has a good chance of moving from host to host. The after-effects of the drug-seeking behaviour itself will soon perpetuate the state that benefits the virus.
Interestingly, some nutriceuticals that ease HCV pathology have also been shown to decrease drug seeking behaviour and related pathologies:
Milk Thistle as effective as fluoxetine for OCD:
http://www.naturafoundation.co.uk/?objectID=4031&page=1
Niacinamide/nicotinamide a GABA (or benzodiazepine) receptor enhancer:
http://www.sciencedirect.com/science/article/pii/0091305780902361
NAC reduces cannabis use:
http://ajp.psychiatryonline.org/article.aspx?articleid=1184217
Does the virus modulate behaviour? Does it need to? And did it do so differently in its evolutionary past?
Before the 20th century, medicine was of less use to blood-borne viruses like HCV. (see the second part of this article, HCV and the History of the Human Race). Warfare would have been a better way to get out and about, to jump from one tribal group or one city-state to another.
Could a virus make us more aggressive or xenophobic?
Would it need to? Or were we already warlike enough to make the perfect host?
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2661026/
Glutamate and compulsion in addictions:
http://psychiatryonline.org/article.aspx?articleid=17769
Acetyl-carnitine in methadone withdrawal (ALCAR is a GABA agonist)
http://journals.lww.com/clinicalneuropharm/Abstract/2009/01000/Acetyl_L_Carnitine_in_the_Management_of_Pain.8.aspx