 As General Robert E. Lee and his Army of Northern Virginia commenced their invasion of Pennsylvania in June, 1863, the 151st Pennsylvania and the Army of the Potomac likewise moved north to repel them.As the campaign began, the regiment was struggling through an epidemic of typhoid fever and dysentery, leaving over 100 men and over half the regiment's officers too ill for duty.On June 8, Col. Allen was granted a furlough due to illness, and Lt. Col. McFarland assumed command of the regiment.