"Dr. Lida Mattman, a fungus expert with Wayne State University, asks the question in her excellent book Stealth Pathogens, "Are fungi in blood smears overlooked because they are confused with blood cells?" Her answer is a resounding "YES". Think of how many blood conditions are misdiagnosed because machines and laboratory technicians can't tell the difference between fungus spores and cells..."
Additionally, what about blood parasites that are known to exist in the blood? They don't get attacked.