On the far left-hand side, that bar represents a group of people who are uninfected, and as we move towards the right-hand side, these people have become infected and they're moving towards the stage that they're infectious.
A la izquierda, la barra representa un grupo de personas que no estaban infectadas, y si nos desplazamos hacia la derecha esta gente ha sido infectada y están llegando al estado en el que son infecciosos.