Can I say, "Bacteria is a virus"?, and can I say "It is a bacterial infection" for the infection which is caused by bacteria.
Are you speaking grammatically or biologically? Bacteria and viruses are different creatures.

Grammatically, you have set up a lose-lose situation for yourself with your sentence. Choose instead either A bacterium is not a virus or Bacteria are not viruses.

It is a bacterial infection -- This is correct...unless it was caused by a virus.
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Thank you Mister Micawber

I know, virus is an organism, so now I think "bacteria" is also an organism. Am I right?

I know we say "It is a viral infection", if the infection is caused by virus. So I guess we say "It is a bacterial infection" if the infection is caused by bacteria. Am I right?
Let us be clear, Gary. These are grammatically and biologically correct:

Bacteria are organisms.
Viruses are organisms.
A bacterium is an organism.
A virus is an organism.
Paris Hilton is an organism.