There's no such thing as a "head cold" - it's a sinus infection. Most commonly it's a mold infestation, easily cured with oregano oil.

There's no such thing as the "stomach flu" it's either a bacterium or a virus, most commonly the Norovirus.

This is the common cold, acute rhinitis.

Influenza is a family of viruses that mutate every year. Some are minor, some are quite dangerous and have killed a significant portion of the globe. Most pathogens are very host specific, they adapt to one very specific host, even dogs and cats (and squirrels)have different types of fleas. But, other animals also have influenza and the virus can and does jump back and forth between species mutating as it goes.

Birds suffer only alimentary distress from influenza, in other animals it's the respiratory sytem that is attacked. While it's true to day birds evolved an immunity to flu you have to understand this is because all the birds that were susceptible to respiratory symptoms died out.