The extracellular matrix, mainly composed of collagen, is a mechanical barrier against infective agents, including viruses. High glycine availability is needed for a healthy collagen turnover. Glycine produced by human metabolism is much lower than the cell's needs giving a general glycine deficiency of 10 g/day in humans. This effect was tested for three years in 127 volunteers who had virus infections usually once or more times every year. 85 of them took glycine 10 g/day; 42 did not take glycine. Among those who took glycine, only 16 (12 of whom had infections two or more times each year) had the flu just in the first year -but much reduced in severity and duration- while those who did not take glycine, were infected as often and as severely as before. Glycine intake at the afore-mentioned dose prevents the spread of viruses by strengthening the extracellular matrix barriers against their advance.