Hi Lisa,

I'm not sure what is causing your milkweed to decline. 

Have you been watering it? 

The purple color on the leaves makes me think a nutrient deficiency, but I don't think that would cause the stems to collapse. Sometimes a nutrient deficiency can happen if a plant gets too much or too little water. I wouldn't advise fertilizing it. I would keep monitoring it. 

I am wondering if the stem collapse is due to Rhyssomatus annectans, a milkweed weevil. Here is an article from Illinois on milkweed weevils. It has some photos and says "This damage to the stem causes the upper stem and leaves of the plant to wilt and break off. Hatching larvae create meandering tunnels through the pith, and numerous older larvae consume the entire pith." Maybe you can go out at night and look for weevils? 

I don't think you need to worry about whatever it is going on transferring to your other plants. Oftentimes, diseases and pests are host specific and will not move to plants from another genus. 

Look for weevils at night, but other than that, this might be a "wait and see" situation.