Dengue is a viral infection which goes through three phases of transformation where it dissipates itself where the person goes back to his good health. Each of these 3 phases is marked by clinical and lab investigation oriented changes, that helps the medical staff to identify the degree of attention required in managing the patients.
This is the period where the patient has fever. This is the phase where the patients are most worried about. This period may last about 3 to 6 days. The duration of the febrile phase is dependent on the type of dengue virus and the immunity of the patient. Most think that the fever is the most dangerous part of the illness, but it is not.
When the virus enters the body and interacts with the blood, progeny are developed which causes fever. This period, itself is not harmful and in some this period may end with convalescence whereas others will enter the critical phase.
This period is marked by a drop in the platelet counts with a reduction in the total white cell count. The reduction in the platelets is significant but not drastic.
This phase entails the most dangerous part of the illness. By this time, the fever has settled and people think that they are out of the woods, and that it is safe to continue with their day to day activities.
By this time there is a rapid drop in the platelet count in the full blood count. This is due to the antibodies ( some due to antibodies of a previous dengue infection)and other immune mediated reactions which cause destruction of the platelets.But the reduction of platelets itself is not the worst of it. It is the leakage of fluids from the small vessels into the surrounding tissues. This is again due to the immune mediated reactions of the body.If you think of the blood like a lubricant fluid which maintains the smooth function of a vehicle, this lubricant fluid contains two components; the watery component (plasma of the blood) and a debris component (cells and platelets). When the tubing of the vehicle is not resilient enough and in developing small pores, in turn leads to extravasation of the liquid out of the tubing (vessels) in to the surrounding parts. So in effect this leads to thickening of the lubricant fluid.