The primary targets of many viruses are the cells of the most sensitive tissue developing the viral disease, such as in the case of SARS-CoV-2, the cells presenting the ACE2 (for Angiotensin-Converting Enzyme 2) receptor, or in the case of HIV, the cells most infected by the virus, i.e., immune cells CD4+ T cells and macrophages, as well as cells of microglia. Here, CD4 is linked to viral load.