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.
Both standard deviation and mean deviation are measures of variation (spread from a central value like mean) in data. It is the mean/average of absolute deviations of data point from mean as suggested by name i.e. we subtract the mean from each data point; take it's absolute value (non-negative); sum it up and divide by the number of observations.