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In contrast, those that had been exposed to the seasonal flu beforehand had virus only in the respiratory tract — and at pretty low levels.

In an update on Monday, the Vatican said the pontiff has a "polymicrobial infection" of his respiratory tract, which has required a change in his treatment.

It’s a particularly exuberant tortoise nicknamed Typhoid Mary, who got the nickname because she harbors a contagious bacteria that causes upper respiratory tract disease.

Viral genome sequences indicate the teen was infected with the type of H5N1 typically found in wild birds and that it had mutated to better cleave to the respiratory tract.

However, the vaccines fail to clear bacteria from the upper respiratory tract, allowing even vaccinated individuals to spread the disease.