Final answer:
The Human Immunodeficiency Virus (HIV) targets and infects helper T cells (CD4+ T cells) and macrophages, causing a reduction in CD4+ T cell counts and weakening the immune system.
Step-by-step explanation:
The virus found in the blood that infects certain T lymphocytes, particularly helper T cells (CD4+ T cells), and possibly macrophages is the Human Immunodeficiency Virus (HIV). HIV targets these cells by binding to a receptor on the cell surface, gaining entry, and then using the cell's genetic machinery to replicate. This replication process can lead to a dramatic reduction in the number of CD4+ T cells, weakening the immune system and increasing vulnerability to opportunistic infections