I need a Combobox with navigation controls

As has been mentioned in the comments, a combo box is not the way to go. There is just too much work for the user trying to page down through 1000+ entries. I don't know if your selection is going into a grid cell, or you are using a full screen, but either way I'd recommend that you allow your user to do selection using a grid. When they click to select a new value, bring up a simple form with a grid on it. That will allow them to see more than just the value they are selecting (which will help them make a better choice) and will allow them to sort and search on different properties associated with the disease.

Once they make their selection, just pop it into the grid or control on your primary screen.

If there is some reason that you feel you *must* use a combo box, then you need some way of getting your data in bite sized chunks. For example, if you decide that 50 items is the maximum that you want in your combo, use a "top/first 50" to get your first chunk of data. Keep track of what the index value is for the last record.

For example, let's say you are getting them indexed on a numeric ID. You get the top 50 records and the last record is #50. Now, in order to get the next "chunk" of data" you query the DB for "top 50 where ID >= 50 sort on ID" (you want to include the previous value in the subsequent pages, see below for reason). And so on.

Now you need something to trigger the pagination. Maybe when the user selects the last value in your current 50 "page", you could query for the next 50 and rebind to the combo box (with the same #50 selected), so that they could scroll to the bottom, select, then select again for the next 50.

Really clunky and I wouldn't recommend it, but there it is.