Even running up a set of stairs will cause your body to release millions of PMNs (polymorphological neutrophils) into your blood stream.
These wee white blood cells are the first line response in the acute immune response, and if you ever wondered what pus was, well it is spent neutrophils.
While training can increase your immune system, overtraining will suppress it.
The immune response is needed for muscle growth, however, too much, it can become a bit overwhelmed, and you become susceptible to infections.
If you have shingles, I would take a break from training.
Shingles arise from one of the herpes viruses, varicella zoster, which causes chicken pox. Like other herpes, once you have the virus, it can lay dormant in your system, and be reactivated when the body is stressed.
You can pass this herpes onto others.
It is more typical in people who have had chicken pox, those over 60, people who have HIV/AID, or those who are taking steroids.
Depending on how long you have had the symptoms, some anti-virals will help you get over it (usually a few weeks), so zovirax or acylovir, but they are most effective within 72 hours.
These drugs are nucleoside analogues, meaning they are 'fake' nucleotides, so the viral mechanisms that have taken over your cell can't assemble more virus cause these nucleotides have a messed up binding site.
Corticosteroids will also relieve the symptoms (prednisone), and some pain killers, but these are just to relieve the symptoms, not a cure.
Unless you rest and let your body heal itself, you could develop serious complications from this virus.
It is a sign that you should listen to your body and REST.