When discussing Egypt it is necessary to point out that Egypt was the epitomy of immorality. Egypt represented everything physical and the society was steeped in immoral acts and ways of living. Egypt was darkness! Egypt was the world's darkness. On the contrary, we see the Jewish people as the "light" ("la'yehudim haysa ora"). And so, just like during the plague when the Egyptians had only darkness, the Jews had light and could see clearly. So we see that it is our job to be the light in the world - how do we 'give off light'. It is every Jew's obligation to glorify G-d's name. Practically speaking, every Jew must act in a way that people look at him and see that there is a G-d and this person is a personal representative. On a grand scale this might mean to go out and give millions of pounds to charities for the world to take note, but on a more real level it is the smile that we give to a stranger, or a thank you that we give to the postman. The effects of which we will never fully know in this life - but after 120 years when 'all is revealed' and there is no limiting body - every moment of life is shown and all consequences are realised.
Therefore, that one kind word that may have cheered someone up will have untold consequences. Perhaps that person became more happy and is nicer to her children on that occassion, they in turn are happier and are nicer to their friends etc etc - this is a very simple analogy - but with a bit of imagination the message is there to take and projected to an unimaginable range of changes in the world. they say that 'a butterfly can flap it's wings in Spain and cause a wave in China', well one kind word can quite literally reach the other side of the world and have a massive effect on the whole future of the world!
We are the the light bearers, it is up to us to light up the world with our Torah and the way we act - only then can we say that we too are not affected by the darkness that engulfed Egypt.