There are some commercial mixes which include ingredients meant to be beneficial to the immune system and the health of good bacteria in the gut and what not. It's always a good idea to feed nutritious, fresh foods. Many berries and small fruits are bursting with vitamins and antioxidants, such as black current and cranberry. One thing my syrian loves is dried goji berries, which are supposed to be rich in vitamins such as a wealth of Vitamin A and C, antioxidants, essential amino acids, loads of trace minerals and are very fibrous. It's a little super fruit, and my Fin loves them! A couple of these once or twice per week in one of his favourite treats,
Some people may recommend a nutritional supplement, but providing that you use a high quality complete mix this shouldn't be necessary. Sometimes when buying a new hamster I give them a small course of vitamin/mineral supplement added to their water to give them a bit of a boost, but I don't keep them on it.
One thing I always feed is a high quality puppy kibble. The one I use is called Nature's Way which is a brand owned by the shop when I work, and I get little hamster sized samples for free which lasts me ages. It's rich in protein, which is great for young or frail animals, and has loads of added vitamins and trace elements, herbs and oils for a healthy immune system, gastrointestinal health, coat/skin and overall health.
Of course, puppy kibble is so nutritionally dense that it may throw off the overall intake of the hamsters diet, so I restrict them to one piece of kibble per week in their bowl. They are also great for wearing down teeth.
I have actually never had to take a hamster to a vet, and all of mine have lived long and healthy lives. Of my litter of pups, all survived and grew to be very active, chunky and robust hams.
I couldn't speak highly enough of a good puppy food, as well as other foods such as porridge, bread soaked in milk, scrambled egg and fresh fruit and veg.