In contrast to tumor-forming solid cancers, “liquid” cancers affect your blood, bone marrow, lymph, and lymphatic system and often start in your bone marrow, and what basically happens here is that the superhighways of your vascular and lymphatic systems are overwhelmed and crowded out by useless cancer cells. (Liquid cancers still are made from cells, they are not actually liquid.) Leukemia, or blood cancer, is often used as a sort of catchall name for these kinds of cancers.
