<unk>  is defined as platelets of less than 150 x 103/mcL.  Symptoms such as bruising and petechiae usually occur at counts at 50 x 103/mcL and between 5-10x 103/mcL there is a high risk of spontaneous bleeding.
Thrombocytopenia
Marrow infiltration 
Thrombocytosis
Viral infections 
Neutropenia
Marrow suppression 
Leukopenia
Congenital thrombocytopenia