Think Taiwan.
Perhaps because that is what they had for their experimental subjects.
More likely, this virus was a research experiment with some "minor modifications" to target specific lab animals as a proof-of-concept. I would believe it is unlikely to be a finished bioweapon.
And then there was an "oops" in the lab.
"Now you know it works. Any questions?" (Stephen King, The Stand)
In the era of nuclear weapons, 'plausible deniability' becomes extremely important - if it becomes known that you've attacked another country, you've placed the existence of your own country at risk. Biological weapons targeting certain ethnic or racial groups may therefore become attractive to some governments (including the Chinese Communists), because unlike nuclear weapons, fatal diseases are not necessarily human-engineered, nor is the source of such diseases as easy to determine as a massive nuclear attack.
A bio-weapon attack, as typically envisioned, would be preceded by the development of a single biological agent, followed by mass immunization of the attacker's own military & populace. Such immunization efforts are obviously subject to discovery by the target country; and the sudden appearance of an agent that affects a specific ethnic group might prompt suspicions as well.
Theoretically, a different approach might entail fewer risks. The attacker might develop two related agents, rather than one, via genetic engineering or a long-term program evaluating natural mutations/variations. The release of the less-fatal strain of the agent in the attacker's own homeland might provide some immunity to the more-lethal variant, and therefore substitute for a traditional immunization program. More importantly, the initial outbreak in the attacker's homeland (with associated death toll) would provide plausible deniability, when the more-letal 'mutation' (i.e., second agent) subsequently infected the populace of the target nation. The attacker might even offer "assistance" to the target, based on their experience 'fighting' the initial outbreak...