Full text can be accessed at https://books.google.co.tz/books?hl=en&lr=&id=XejcAwAAQBAJ&oi=fnd&pg=PA113&dq=Nonstandard+Discretizations+of+the+SIS+Epidemiological+Model+with+and+without+Diffusion&ots=XiptlZ0CvW&sig=h-yW33XtuhTK-WceKRrZna-9I5w&redir_esc=y#v=onepage&q=Nonstandard%20Discretizations%20of%20the%20SIS%20Epidemiological%20Model%20with%20and%20without%20Diffusion&f=false