Colds, Flus, Bacteria Viruses and other Mean Things
Perhaps these issues may have a longer range of impact on the human body than just the acute or active stage that a disease may be in. They can disrupt or weaken functioning and one's immune response.
In my experience viruses such as Mononeucleosis and Lyme's disease can have longer term disruptive effects on the body's immune system as well as various body system problems like migraine headaches, chronic aches and pains, fibromyalgia, chronic fatigue, and so on.