An epidemic is an     1     disease that spreads rapidly to a large number of people within a short period of time. Epidemics have been occurring as long as humans have been on Earth. Some epidemics are     2     of previous viruses.

A disease reaches epidemic levels when a certain number of deaths occur as a result of it and this number will be different for each disease. One disease might result in just a few deaths but be classified as an epidemic, as     3     others such as the flu, which is     4    but has much higher baselines.

In human history, there have been some deadly epidemics. In the early 20^th century, the Spanish Flu of 1918 was a massive epidemic that killed an     5     of 50 million people, more than the number of people who died in World War l. Initially, it was believed that epidemics     6     little threat to the modern world, until SARS outbreak in 2002. The SARS epidemic started as a     7     disease in China but eventually spread across the     8     to countries such as Canada and Vietnam. It resulted in the infection of about 8,000 people and since then, scientists have undertaken     9     on treatments to prevent future deaths.

Su Xiao, 49, and Xu Guangchun, 42, are like-minded souls on the streets of Beijing, checking surveillance cameras and     1     passersby, with a near-constant barrage of calls on their phones.

They are on the lookout for senior citizens with Alzheimer's disease, a hard-hitting disease that can easily     2     a patient's memory and other major mental functions.

Seven years ago, Su and Xu co-founded the Beijing Voluntary Emergency Rescue Service Center, which     3     a public welfare campaign to help families find their lost elders in 2016.

Su, an outdoor sports lover, is a seasoned mountain rescue     4    . On New Year's Day of 2016, he was on his way to a nearby ski resort where he ran into a listless and pale elderly woman who was holding a sack and     5     in the cold under a bridge.

She was mumbling, saying that she was about to buy noodles for her son, and this    6     Su of his grandmother, an Alzheimer's patient.

Su reported the situation to the police and they found a scrap of paper with a     7     number in her pocket. It turned out that days had passed since she lost contact with her her family and she had traveled more than 40 kilometers from her home in southwestern Beijing to an unfamiliar neighborhood in the east of the city.

According to statistics     8     by a white paper released in 2016 by the Zhongmin Social Assistance Institute, about 500,000 senior citizens get lost each year, with about 80 percent over the age of 65. Alzheimer's disease is one of the top reasons for them going missing.

Su and his rescue team watch surveillance videos first to sort out clues before further rescue efforts, and rely on the elderly person's experiences in their childhood and youth as clues when looking for them.

He once managed to find an 80-year-old along a river in a suburb of Beijing,     9     on the elderly person's childhood life experience of living on a riverboat.

The youngest person they have found was in their late 40s, Su said, adding that patients aged under 60 are difficult for family members and other people to spot, not to mention those who dismiss the illness as ominous and are     10     to confide in their neighbors, relatives or friends.

Su and Xu's "lost and found" service has sent more than 320 elderly back home safe and sound. More than 500 volunteers, including some family members of people they have found, have joined the rescue team.

"The farther we walk, the closer the lost elders get to their homes," Su said.