B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 250; Flight Crew Type 2500
Situational Awareness
ATC Issue All Types; Conflict NMAC
General None Reported / Taken
Human Factors; Procedure
We were vectored off of the arrival (ENDEE 4; I believe) on a left base and told to expect the visual 22L. The Controller gave us a left turn to 340 degrees and a descent from 4000 ft to 3000 ft. Passing through 3250 ft and inside of SAILZ approaching final; the Controller called out a traffic alert and told us to level off. We leveled at 3150 ft. He then told us to make a tight left turn towards the airport. He called traffic off our right side which we were unable to acquire visually. I immediately turned the autopilot off when told to make the tight left turn and was flying mostly by instruments and monitoring bank angle and altitude and thus unable to scan outside for traffic. The Captain was looking outside and unable to see the conflict. I did look down at my MFD and saw a yellow traffic target on the right side of our aircraft symbol at +100 feet.We were subsequently cleared for the visual approach once the traffic was no longer a factor and landed the aircraft normally. Oddly; we did not receive a TCAS RA or TA aural alert. 100 ft of vertical separation is the closest I have ever been to another aircraft and consider this a near miss event.There were no crew errors made which contributed to this event. We followed all ATC instructions as precisely as possible. I believe this was an ATC Controller mistake who gave us a bad vector/altitude which led to a near miss event. It is also possible that it was not an ATC mistake and the GA aircraft was at fault and in airspace which he should not have been. On a separate note; it is my opinion that GA aircraft should not be allowed to fly so close to congested Class B and Class C airspace. An overhaul of GA rules would prevent this issue. Also; if GA aircraft are allowed to fly so close; they should be required to be in constant contact with ATC.
B737-800 First Officer reported an NMAC on approach to MDW.