In the lungs, for example, swelling can prevent oxygen from reaching your blood vessels. Oxygen normally moves across very thin membranes in endless moist chambers inside the lobes of your lungs. When that tissue becomes swollen with immune cells, it's much harder for oxygen to reach the tiny blood vessels that it's supposed to move into. There's also some degree of damage to the lung tissue. Viruses can't replicate by themselves, so they have to do it inside your cells instead. That means that in order to kill the virus, your immune system has to destroy your body to some extent, which only makes oxygenation harder.