In this paper, a multilayer/monolayer black phosphorus (BP)-based nanostructure is presented to detect the avian influenza virus. The nanostructur is a grating arrangement made of BP over SiO2 or Al2O3 substrate. To achieve the transmission spectrum, depend on the changes in the lateral length of BP, namely (L = 100, 150, 170 nm) as well as the complex refractive index of each of three viruses types (H1N1, H5N2, H9N2) in the THz range, the structure is numerically simulated by 3D Finite Difference Time Domain (3D-FDTD) method. The change in resonance frequency is greater for the H9N2 virus because the real part of its refractive index is relatively larger. Here, too, the rate of change is examined based on the different thicknesses of the H9N2 virus. Also, changes in the refractive index of the environment have been used to calculate important parameters in the sensors, such as sensitivity, FWHM, and figure of merit. Overall, this platform provides a promising platform for detecting influenza viruses.