What is the reason for the virus flooding in the movie dawn of the dead?

[Edit this paragraph] The story introduces a terrible plague that swept the whole United States. After the whole world was attacked by the plague, millions of bodies were infected and turned into vampire. Dead or alive, as long as a zombie sucks blood, it will immediately become a zombie. Soon, the whole city became a zombie city, and human beings suffered unprecedented looting.

Only a few people survived the double disaster of plague and zombies, including a young nurse Anna (sarah polley) and a police officer Kenneth (Vin Ramses). They hid in a shopping mall in a medium-sized city in Washington state and carefully protected themselves from zombies by using hidden containers.

However, zombies still have some memories of human beings. People like to go to supermarkets and may smell their blood circulation. More and more zombies came to the shopping center, and the few who survived could hardly hide in the end. They must find a way to get out of here and find a really safe place. At the same time, they have to overcome their own contradictions and unite against zombie attacks.

Led by nurse Anna and police officer Kenneth, the survivors plan to go to an island that may not be attacked by plague and zombies in order to survive.

It should be a plague, but there is no explanation in the movie about why there is a plague.