I can explain the general concept, up to you to devise the rest.
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Here's the basic premise: You have a piece of wood firmly attached to the ground, and a piece of wood firmly attached to something that's bent down (typically a tree). You bend the tree down and attach the tree hook to the ground anchor, and tie a net to a tree.
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The most direct way of doing this is to cut a square notch in the tree hook and/or the ground anchor so you can clip the two pieces together. When the animal runs past and bumps into the hook mechanism the two pieces pop apart, causing the tree to suddenly stand up and lift the net up with it.
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This is great for small game because a small sapling is bendy yet still capable of lifting up a small rabbit. For catching a human however a sapling won't do the trick. You could use the same idea to cause the net to drop, or alternatively use a weight/pully system to have a weight heavier than your target applying pressure to the trigger (though for this you'll likely need an industrial cut 2x4 or metal bar, and the human will have to run into it fairly hard.
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Another method would be to have a thin nylon cord as a tripwire to pull a pin out of something. You could say have a wooden board that just barely doesn't reach between two thick branches of a tree, then using the "pin" (maybe a book or a bottle or something) provide the bar with the extra distance. When the cord is pulled out, the wooden bar will drop, releasing whatever weight or net you want.
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The key is figuring out basic spring traps, once you get the hang of that you can make all sorts of stuff.

Here's another idea: Use a closet pole (the round 1 inch diameter 10 foot long poles) as a tripwire, run the tripwire diagonally from one corner of a hammock to the other, and place it so it's forming a diamond shape on the persons path, then attach two smaller weights to each of those corners running up and over a tree branch. The corner the person enters from will be laying flat, and when the bar is tripped the net is free to lift.
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I did a quick sketch to illustrate the idea.

wait just a damn minute... why are you trying to CATCH a live zombie again?

I thought that would be obvious. Experimentation. That way, you would have fresh blood and tissue samples to study the virus, or whatever made the zombies, and you could use the zombie as a test subject. See if it's curable, if the zombification process could be reversed.

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you're a biologist with experience handling contagious viruses? i had no idea. im not so im staying as far the fuck away from them as possible

Not yet. As I stated at some point (I believe on the old forum), I am going to school to be a microbiologist. I'd like to work for the CDC. If an outbreak were to begin tomorrow, there would be other reasons for capturing one alive. Ever see the movie 28 days later? I think it would be nice to know how long the zeds would be able to survive without food, so as to have a projection date for when it's "safe", at least from the zombies themselves.