Most people have frequent "colds" which are usually caused by rhinovirus, adenoviruses and others small viruses - the body usually confines these to the upper respiratory tract and usually fights them off in 2-5 days.
Over 5-10 days the flu
can spread down into the lungs, causes high fevers, muscle pain
, joint pain
and more rarely lead to severe pneumonia
requiring ventilator support and death.