Our blood comprises of red blood cells, white blood cells, platelets and plasma. The function of platelets is to repair the vessels that puncture or tear, so that the blood does not ‘leak out’. They do this by forming a mesh type structure over the damaged vessel and retain the blood to flow within the vessel itself. Think of it as how you seal a pipe by applying an epoxy. Upon noticing a tear, they get ‘activated’ and start forming a mesh.
Once a person is affected by a dengue virus, the platelets can no longer ‘activate’ itself. So whenever there is damage, the platelets cannot form the mesh structure and the blood flows out of the vessel. As blood flows out, so does the platelets. This causes the platelet count to fall drastically. The blood that flows out causes internal
and external bleeding and leads to a haemorrhage.