When they prevented germ-free mice from making a receptor on the surface of their cells, infection with norovirus didn’t lead to an improvement in their guts.That receptor only latches onto one type of molecule. It’s called Type 1 interferon, and it’s produced by cells when they’re invaded by viruses.

Unlike the high particle-infectivity ratio of 106 to 108 previously reported for these viruses, the number of total particles per PFU ranged from 160 to 490 with strain AD-169 and from 176 to 1,050 for strain C-87.

A high particle-to-pfu ratio is sometimes caused by the presence of noninfectious particles with genomes that harbor lethal mutations or that have been damaged during growth or purification. Another explanation is that although all viruses in a preparation are in fact capable of initiating infection, not all of them succeed because of the complexity of the infectious cycle. Failure at any one step in the cycle prevents completion. A high particle-to-pfu ratio does not indicate that most particles are defective, but that they failed to complete the infection.