<span>A person in the young adulthood stage is between the ages of 21 and 35. Young adulthood is a time when growth rate slows and people reach peak physical health. People in this stage generally make more responsible choices, have a better sense of identity, and experience less conflict. This is also the stage in one's life when one selects a job or career path, enters a committed relationship, gets married, and starts a family. A person in the middle adulthood stage is between the ages of 35 and 65. This stage of life is characterized by a period of security, satisfaction, and maturity. People in this stage often experience diminished muscle tone and strength, and women go through menopause. Also, people in middle adulthood may pursue new challenges and reflect on their mortality. Major life events that occur in this stage may include focusing on career and family, experiencing a midlife crisis or the empty nest syndrome, and acquiring new illnesses.</span>
Adaptive immunity can be acquired during either 'naturally' (by infection) or 'artificially' (through deliberate actions such as vaccination). Adaptive immunity can also be classified as 'active' or 'passive'
