In ancient past the only way for people to communicate was to talk to each other - in sign language, or through pictures, or through a proper language. Conditions improved when we discovered script and ways to write down words. Printing press made it absolutely easy to create multiple copies of what we had to say. But computers connected to the internet changed the scenario.
With the ability to copy-paste and spread a word without spending a penny came a detestable habit - forgetting to read and analyse what is going through. While copy-pasting needed one to define one's selection by dragging with their mouse around the right words, "Share" or "Forward" makes it a single-click affair, and much worse.
Here is an audio (in Hindi) which tells us how this ease-of-use has actually made us lazy to think about even the consequences of a click. A radio jockey pretends to be a Facebook manager who is rewarding active users and calls up two guys - a Muslim and a Hindu - who had put status updates against each other on the preceding day. When asked about what they had posted, they are ashamed to answer.
This happens in chain forwards too. Most chain forwards begin with a claim of authority ("The Scientific Association of ABC warns that...") which is directly followed by a false claim ("...eating XYZ is harmful for body as it contains UUU..."). Following this there is either an unscientific explanation of how this works, or a very emotional story of someone being affected by not following this advice. And at the end there is the quintessential "forward this to your friends and relatives".
The people who actually read this critically will focus on two things - the authority who is claiming it, and the explanation behind it. They proceed to verify that the authority has actually issued such a warning or that the purported harm is plausible according to the given explanation.
But the people who forward it mercilessly focus on different things - the claim, the emotional story, and the "harmless" opportunity to help a lot of unsuspecting people. I wonder if they even read the claims. They gloss over the long piece of text and then think "Uh, oh! What if this is true? This affects a lot of people, I suppose. After all there is no harm in just sharing it." Click. Shared.
The people in the above audio clip might also have done the same. They would have superficially understood the emotions being conveyed in a message. And then they would have taken it for granted that the claims in it are actually true. And, worst of all, they would have shared the message with best intentions.
It is when we forget to slow down and think that mistakes tend to happen.
When in doubt, do not share. When you are compelled to share, "say something about it". Add your opinion, or doubts about it along with what you share. And never share something that you have not read fully.