Final answer:
The white blood cells in the blood are affected during an infection, as they are the body's primary defense against pathogens. Bacterial infections can cause phagocytes to respond, while viruses like HIV can weaken the immune system by killing white blood cells.
Step-by-step explanation:
If someone has an infection, the layer of blood that would be affected is the one containing the immune cells, particularly the white blood cells. During an infection, the immune system does not retreat, but rather, it is actively involved in fighting the pathogen.
In the case of bacterial infections, phagocytes in the blood engage the invading bacteria to prevent their unrestricted spread which could lead to serious complications.
For viral infections, like with the Human Immunodeficiency Virus (HIV), the virus infects and kills white blood cells, which eventually weakens the immune system's ability to fight off pathogens and increases the risk of illness.