In cases of immune system over activity, the body attacks and damages its own tissues (autoimmune diseases). Immune deficiency diseases decrease the body's ability to fight invaders, causing vulnerability to infections.
Standard deviation is a measure of spread (like Range, Variance, or IQR) so it tells you how wide or concentrated your data is. You can interpret it as how far away the typical/average observation is from the mean.
Because the definition of standard deviation is, essentially, the Mean distance the data points are from the arithmatic mean of the set. So a set of (2,4,6) has an arithmatic mean of 4, and a standard deviation of 1 1/3 ((|2-4|+|4-4|+|6-4|)/3).