Life expectancy is a measure used to gauge the overall health of a population. It is the average number of years of life at birth that an individual could expect to live if current death rates were to remain constant and no one moved in or out of the population. Shifts in life expectancy are used to describe trends in mortality. Life expectancy at birth is strongly influenced by infant and child mortality, improvements in nutrition, housing, hygiene, medical care, prevention, treatment and control of diseases throughout the lifespan. These factors contribute to decreases in death rates and increases in average life expectancy. Migration into and out of a population may also influence life expectancy statistics by increasing or decreasing death rates in specific age groups.