Certain viruses spontaneously assemble from hundreds to thousands of subunits with stoichiometric precision. Viruses also can undergo large mechanical deformations after which they spontaneously reconstitute to the initial morphology and function. Both properties, stoichiometry and reconstitution, remain very difficult to emulate in synthetic materials. I will discuss attempts at shedding light on the physical principles behind these properties by studying assembly and deformation in presence of mechanical constraints for two systems: the immature Human Immunodeficiency virus and the Brome mosaic virus.