Provided are cellulose microfibers (fibrous cellulose) that can be used in a general-purpose manner for a variety of applications and that have a uniform fiber diameter distribution and a low proportion of both cellulose nanofibers and coarse structures, a composition containing said cellulose fibers, and a porous body composed of said cellulose fibers. The present invention relates to cellulose fibers in which the average fiber diameter in a dry state is 0.3-3.0 um as observed using an electron microscope, the proportion of fibers having a fiber diameter of less than 0.1 um is 5% or less, and the proportion of coarse structures as indicated by the proportion of fibers having a wet fiber diameter of 20 um or greater measured through automatic optical analysis is 3.0% or less. The present invention also relates to a composition containing said cellulose fibers, and a porous body composed of said cellulose fibers.

A virus removal membrane includes cellulose, and a primary-side surface through which the protein-containing solution is to be applied and a secondary-side surface from which a permeate that has permeated the virus removal membrane is to be flowed, wherein a bubble point is 0.5 MPa or more and 1.0 MPa or less; and when a solution containing gold colloids having a diameter of 30 nm is applied through the primary-side surface to the virus removal membrane to allow the virus removal membrane to capture the gold colloids for measurement of brightness in a cross section of the virus removal membrane, a value obtained by dividing a standard deviation of a value of an area of a spectrum of variation in the brightness by an average of the value of the area of the spectrum of variation in the brightness is 0.01 or more and 0.30 or less.

A virus removal membrane includes cellulose, and a primary-side surface through which the protein-containing solution is to be applied and a secondary-side surface from which a permeate that has permeated the virus removal membrane is to be flowed, wherein a bubble point is 0.5 MPa or more and 1.0 MPa or less; and when a solution containing gold colloids having a diameter of 30 nm is applied through the primary-side surface to the virus removal membrane to allow the virus removal membrane to capture the gold colloids for measurement of brightness in a cross section of the virus removal membrane, a value obtained by dividing a standard deviation of a value of an area of a spectrum of variation in the brightness by an average of the value of the area of the spectrum of variation in the brightness is 0.01 or more and 0.30 or less.