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.
A standard deviation much larger than the mean is certainly possible. It could be, for example, that your data are highly skewed. the simple dataset 42, 50, 55, 999. and the dataset has a mean of 287 but a st.dev of 475. Standard deviation is a measure of the spread of your data. of how peaked your dataset is).