Second Childhood is a disease in the Pegasus galaxy which acts like a fast onset equivalent to Alzheimer's disease on Earth. It is caused by a parasite that embeds itself in the brain and slowly shuts down the cells around it as it expands. Those affected begin to lose long-term memory in mere days, and within two weeks become quite childlike. This causes loss of memory, motor functions, and eventually major organ shut down. It is very common among the elderly, as those with stronger immune systems are able to resist infection.
The parasite is sensitive to ionizing radiation, which causes it to contract in order to protect itself. This in turn allows those afflicted to regain their memory and motor skills while exposed to it. However, as ionized radiation is also dangerous to humans, this treatment is only effective for about a day, after which the body's immune system fails and the victim dies of secondary infection. Leaving early is also fatal: once the radiation is no longer present, the parasite rapidly expands to its original size, killing the victim in short order. People in the Pegasus galaxy often take their afflicted loved ones to the Shrine of Talus, which emits ionized radiation, to spend one last clear day with that person before death.
Dr. Meredith Rodney McKay, the Chief Science Officer with the Atlantis Expedition and a member of the First Atlantis Reconnaissance Team, contracted the parasite during an offworld mission, when a dam broke and flooded the area around the gate with water. The freezing temperatures lowered his immune response, allowing the parasite to take hold. He was eventually taken to the shrine for his last day. Once there, however, Dr. Jennifer Keller was able to operate on him and remove the parasite using some unorthodox surgical tools with assistance from Lt. Colonel John Sheppard while Ronon Dex later destroyed the parasite for good with his blaster. (SGA: "The Shrine")