The virus was initially transmitted to its new host through the optic nerve. Upon infection, the host itself facilitated the metamorphosis the virus needed in order to begin the next step of replication. Its initial structure, in which its genetic information was encoded as a pattern of light in different colors, proved to be unable to survive inside the host's brain. But through a cunning trick of biology, the host automatically recreated the genetic information as electrical impulses once the progenitive form reached the visual cortex. This was unintentional and automatic on the host's part, a flaw in its biological 'security system' that the virus was able to freely exploit. It settled into the frontal lobe and began to reproduce.
Initially, the host was unaware of this process. It allowed the virus to enter through the optic nerve, but it remained under the impression that it had not been infected. It continued its activities, absorbing multiple patterns of light without ever realizing that one of them was different from the rest. Occasionally, as it accessed portions of its frontal lobe already colonized by the virus, it would encounter one of the iterations of the viral structure, but this process was similar enough to the natural retrieval of memory that the host simply thought it was bringing to mind a recollection of the initial lifeform.
This apparent similarity to natural biological processes within the host was compounded by a natural camouflage strategy that the viral lifeform employed. It utilized a structure that was similar enough to other visual stimuli that the brain automatically shunted it to the limbic system as well as the frontal lobe. The limbic system, unable to differentiate between the virus and its own reproductive instincts, responded with sexual arousal whenever the frontal lobe called up a new copy of the virus.
Once the limbic system triggered sexual desire, the entire parasympathetic nervous system responded as if the stimulus was a genuine reproductive opportunity. The host's vaginal canal lubricated, the clitoris began to swell in anticipation of stimulation, and the host's nipples stiffened as well (due to a complex series of interconnected signals from the brain, and not necessarily because of their direct relation to the reproductive process). In essence, the virus hijacked the normal process of arousal and desire to make the host's brain not just amenable to the infection, but actively conditioned to encourage it.
Frequently, the host responded by manually stimulating its reproductive organs, even using tools at times in order to recreate the sensory experience of biological reproduction more accurately. While it did so, it repeatedly stimulated the portions of the frontal lobe colonized by the virus, allowing it to reproduce continuously during those periods. The host believed itself to simply be engaging in memory retrieval, of course. It consciously associated the virus with sexual pleasure, and as with any animal receiving a positive stimulus, it sought to recreate that stimulus as much as possible.
These simulated reproductive sessions frequently ended with the intense release of a number of neuro-chemicals that altered the physical chemistry of the brain, creating conditions more amenable to the virus. It widened the pathways between the portions of the brain occupied by the viral lifeform, and the limbic system that responded to its electric signals. The chemicals themselves had a stimulatory effect as well, causing the host's neural receptors to crave further doses. Although there was no actual 'withdrawal' effect, the host was in effect being conditioned to allow the virus greater and greater leeway in triggering these reproductive simulations. Portions of the frontal lobe that normally held the reproductive urge in check began to wither under the near-continuous sensory onslaught.
This initial stage of the infection played itself out over three to four weeks. Although the host's initial defenses restricted the virus to activation during periods of solitude, usually when the host's resistance was low due to circadian rhythms affecting its biological processes, the virus gradually gained more dominance as the attack on the frontal lobe progressed. As the virus hijacked more and more of the memory process, the host found itself recalling the viral structure more and more often, even in situations where it was trying to bring to mind another association. The host assumed this to be a natural flaw in its mental processes, little realizing that it was having difficulty because the virus was subsuming other memories to make more room for its replication.
The first sign that the virus had reached a critical stage in its colonization of the host's brain came when the host was in a diurnal social situation and felt an irresistible, limbic compulsion to simulate reproduction. The host had a number of learned behaviors in this situation, all of which strongly associated reproducing in its current environment with danger and released appropriate 'fear' hormones at the idea. (Although the danger was abstract and complex, the emotional response associated with it was similar to that of predation.) The host recognized clearly and directly that this was a time to engage in survival activities (again, complex and abstract ones related to acquiring a medium of exchange for food and shelter, but still survival activities), not reproduction.