(CNN) -- At first, Kip Scheidler said he felt the ground tremble.





Then the roof began to shake. Tiles tumbled from the ceiling as people bolted from their chairs to escape. When Scheidler tried to follow them, he could barely keep his balance as the ground wobbled.





"The weird thing is that afterward you will swear that it lasted 20 minutes but it really lasted 60 seconds," Scheidler said of the earthquake tremors. "But at the moment, it seems like it will never stop, and it keeps getting stronger."





That's how Scheidler remembers one of the most horrific moments in his life -- the 1986 earthquake in El Salvador that killed 1,500 people. All those memories came flooding back recently when Scheidler turned on CNN and saw images from this week's earthquake in Haiti, which may have killed at least 100,000 people, according to initial estimates.





"I was sickened by the news," said Scheidler, now the senior director for global disaster response for Habitat for Humanity International, an international relief agency.





"I was there [El Salvador] as family members identified the bodies of family members pulled from the rubble. It's something that you can never forget. I can't imagine hundreds of thousands of people going through that now."





For people like Scheidler, the news from Haiti isn't abstract. It's personal. They are people who have either survived natural disasters or respond to them. The Haitian earthquake has forced some of them to think again about the survival lessons they learned, and what Haitians face in the days ahead.





How disaster brings out best, worst in human nature





Several say disasters can reveal the worst in human nature.





Patrick Johns traveled to Indonesia in the immediate aftermath of the 2004 Tsunami. He's also traveled to Ethiopia to aid in famine relief and Rwanda after the country was decimated by genocide.





Johns, who is now the director of emergency response for Catholic Relief Services, said it is critical to immediately establish security right in Haiti.





"Crime is a problem in Haiti even in the best of times," Johns said. "With this kind of situation, it's going to go from bad to worse."





Security can make the difference between life and death, Johns said. It's not enough to deliver the right supplies to Haitians. Someone has to guard the good Samaritans.





"You could get stopped on the road, your driver could be thrown in the ditch and you lose everything," Johns said.





People responding to the disasters often have to make excruciating choices, Johns said.





In the past, aid workers would help some needy people but turn down others when they ran out of supplies.





How an aid workers makes that decision can cause a riot, Johns said. It's better to talk to people beforehand to warn them that not everyone will get help immediately, but eventually all will be helped, he said.





"If you can only build 30 ramshackle shelters, who are the 30 who get them and what's going to be the reaction of people who don't get help," Johns said. "It can be nightmarish."





See other places hit by historic natural disasters and their recovery





Another disaster lesson: Don't wait for the authorities to help, several disaster experts said.





Tad Agoglia is the founder of the the First Response Team. Since May 2007, he and his crew have aided thousands of victims at disaster sites across the United States, free of charge, he said.





Agoglia said disaster survivors become energized when they stop waiting for the cavalry and start doing what they can within the first ten hours of the disaster.





When people act, they shake off their stupor, he said.





"Things start moving. Roads are opened up. Order comes to chaos and people start to speak to each other," Agoglia said. "People began to have hope. They say, 'It's not going to last like this forever. We can begin again.' ''





Impact Your World:
What are disaster witnesses reflecting on? What does destruction bring out? Who witnessed the disaster? When did the earthquake hit El Salvador? what brings the good and bad in human nature?
the survival lessons they learned, and disasters can reveal the worst in human nature. Kip Scheidler 1986 disaster
question: What are disaster witnesses reflecting on?, answer: the survival lessons they learned, and | question: What does destruction bring out?, answer: disasters can reveal the worst in human nature. | question: Who witnessed the disaster?, answer: Kip Scheidler | question: When did the earthquake hit El Salvador?, answer: 1986 | question: what brings the good and bad in human nature?, answer: disaster