Why we can't consider it is another life-form?

Why must basic of rules of living organism must be born? cannot just based on replicate? Bacteria also replicating, but it is considered a life form.

It is just we define cellular structure is the basic of unit of life. We only understand little on nature works and universe actually.

Yup, it could be alien coming from outer space. That's why I always posted alien is not necessary must a big head, a pair of eye, 2 legs and arms (in alient/UFO) thread.

It could be in virus form as well. Why not?

There are many reason for it.

1. Virus is mutating all the time, just happen now, it become deadly due to its "new". Once the virus getting old, vaccine is produced, immunity is built, eventually we don't see them much afterwards.
2. Human population become denser, air travel a lot, which become good source of medium of spread.
3. Human live in some area that previously not habitat before, aka invasion of natural segregation of species or in other word, invasion of wildlife habitat.
4. In the old day, there may are other kind of virus, due to studies are not intensive and not well understand about virus. People died due to some disease but doctors don't know why, so it is not logged or history it. So when we refer back, there is no record of those event.