In my world, I plan for an alien pathogen to cause drastic mutations in all lifeforms, reshaping plants, animals and humans alike in a scenario of near inversion of positions, with animals and certain plants mutating and achieving human-level brains and with many growing in size, while human minds degrade to something more primitive, not all the way to reptilian brains, but more like that of a more primitive pack hunting mammal. Patient 0 however has different changes.
Patient 0, while actually achieving a relatively more complex and efficient brain, doesn't escape to the "rewiring" characteristic of the pathogen. Rather than what I planned on this question, patient zero (let's call him "A") suffers from a sense of overall detachment towards his previous group, seeing the infected with a desire to to protect and care for (justified both by the viral alterations as well as by events in his life). At the same time, "A" normally sees uninfected humans as one would normally see an ant (complete indifference, at times an annoyance, at times something interesting to watch). "A" also has by default (moment 0 after he mutates) a relatively atrophied sense of guilt and what one could call a "turned off" empathy sensibility.
Important info: "A" was mutated at the age of 15-16 years old, and still possesses the brain structures necessary for empathic behavior. He also still has memories of his time as a normal human and of his empathic capabilities (he didn't have any problems similar to ASPD). However, the virus caused what one could call a "rewiring" on his sense of empathy, essentially making his empathic capabilities towards others, especially the non-infected, to be "optional".
In order words, "A" has both the structures necessary for empathic behavior and memories of a time he was capable of empathy towards humans. Given this scenario, Could "A", based mostly on his memories and the presence of the necessary structures, essentially "learn" how to emphatize with the non-infected again, be it with or without help? The main motivation for him to do so is a relative inner conflict towards how he once saw and now sees his family, friends and classmates, as well as his own view of empathy as an evolutionary advantage regarding the construction of a more peaceful world (plus the mutated elephants still can do it so it bothers him not to).
Regarding what I found in research and personal experience: despite human empathy being considered to decrease with evolutionary divergence time, there seem to be people who demonstrate a higher level of empathy and compassion towards creatures usually not empathized with, such as arthropods, which makes me wonder if "A" too couldn't emulate something similar with a "species" that's already similar to him and to which he once belonged. Regarding research, most of what I found didn't quite answer my question, being mostly focused either on "how to be a more understanding partner in a relationship" or "living life as a psychopath", Although This information got me quite intrigued, and made me wonder whether something similar could apply to my character, and whether he could make use of rational though combined with an "empathy switch" to at least achieve a state of active empathy, in which he will need to be actively trying to empathize with those around him, but still demonstrate something.