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.
Standard Deviations. The Standard Deviation is a measure of how spread out numbers are (read that page for details on how to calculate it). When we calculate the standard deviation we find that (generally): 68% of values are within. 1 standard deviation of the mean.