Influenza viruses enter the cell by receptor-mediated endocytosis. The viruses are surrounded by a membrane that contains a fusion protein, which is activated by the acidic pH in the endosome. Upon activation, the protein causes the viral membrane to fuse with cell membranes. An old folk remedy against flu recommends that one should spend a night in a horse’s stable. Odd as it may sound, there is a rational explanation for this advice. Air in stables ammonia (NH₃) generated by bacteria in the horse’s urine. Sketch a diagram showing the pathway (in detail) by which flu virus enters cells, and speculate how NH₃ may protect cells from virus infection.