Most of us have low blood volume. Once we have low blood volume, if we get IV fluids, our blood volume is increased, however the effect is temporary. We will lose that extra fluid in 1 or 2 weeks.
Based on reading that, I came up with a reason for the low blood volume. When we have the initial trigger, our bodies are under such stress that it needs to conserve energy. It is busy fighting the infections and starts producing less and less RBC, WBC, and platelets. The volume of blood decreases so that we have the same concentration of RBC, WBC, and platelets.
That explanation may be too simplistic. Perhaps we have a reason for not producing as many RBC, WBC, and platelets.
In reading about platelet production, I see that causes of less production can include: If you have autoimmune diseases, chicken pox, cancers, or bacteria in the blood. Another reason is not enough available folate or B12, and many of us have a genetic defect related to folate. Ibuprofen and asprin can reduce production. Platelets can get trapped in the spleen too.
I believe our blood volume lowers over the months after infection, or when our body starts losing the battle against the invaders that keep coming.