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.
Estimates standard deviation based on a sample. The standard deviation is a measure of how widely values are dispersed from the average value (the mean). Important: This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage.