Hey,

Yes,you can be a Virologist after completing yout Btech. But it will need a lot of hard work as both the fields are totally different and you need to study do much in order to become a Virologist.

GET COUNSELLED BEFORE YOU CHOOSE.

HAVE SOME PAPERS PUBLISHED IN VIROLOGY (or whatever it may be)

try to get as many research internships(IIT brand tag can help) based on the research (more projects the merrier ) because they are the key stones for you getting a good research university(cause virology requires systems biology and basic bio researches