T-Dragon

Nakamaru Koutarou, a high schooler and skilled kendo practitioner, has his peaceful life shattered when a mysterious virus with a 94% infection-fatality rate puts Japanese society on the verge of collapse and threatens the whole world. Right when his little brother is infected, he is scouted by an organisation at the forefront of Japan's last ditch-effort against the virus: miniaturise a company of daredevil soldiers to capture the new life form that acts as carriers for the pathogen, so they can be used as a source for a vaccine/antiserum!

(Source: Mangadex)