Dengue fever occurs in tropical and subtropical areas of the world, It is spread by mosquitoes. Four related viruses can cause dengue fever: DENV-1, DENV-2, DENV-3, and DENV-4.
Dengue virus mainly affects production of platelets production. Platelets last about 5-10 days in our body. So if some one is infected from dengue virus then his/her platelets will start decreasing after 3-4 days. Once platelets count decreases to 50,000 or less then risk of bleeding(internal and external) is much higher and This situation is very lethal.
Platelets count test can not confirm that person is infected from dengue virus but low platelets count is symptom of dengue so doctors can do another test.
If someone is suffering from dengue then to monitor platelets count frequent test is required and doctors try to keep platelets count over 50K by inducing external platelets.