Which type of flu primarily affects birds?

The correct answer is H5N1 (Bird Flu). H5N1 is a highly pathogenic avian influenza virus, commonly known as bird flu, which primarily affects various species of birds. While it can, in rare instances, transmit to humans and cause severe illness, its natural reservoir and primary host are avian populations.

• Option A, Swine Flu (H1N1), primarily affects pigs, though it can infect humans. Option C, Seasonal Flu, refers to common human influenza viruses. Option D, Spanish Flu, was a pandemic strain of H1N1 that predominantly affected humans in 1918. None of these primarily affect birds.