They affect us- this refers to a general pattern rather than now- f it were at this moment, you'd say 'they're affecting us'.
We can use the simple present for things that are now, when they are of very short duration or instaneous. If you listen to sports commentaries, say, you'll hear 'he shoots'. This is because the action is so quick that it doesn't have any real duration. With your example off 'affects us', it is unlikely that it would be instant and have no duration.
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