Something serious, or after midnight?! Go to the nearest ER, called a "Pronto Soccorso" (pronunciation: pron-toe so-CORE-so)
Need an ambulance? Use a local phone to call 118.
If the problem crops up during the daytime, and you don't think you need either an ER or an ambulance, Milan's public health system comes to the rescue!
Centrally located in via Rugabella at n. 4 between Corso di Porta Romana and Corso Italia, not far from Piazza Missori (exit: Piazza Missori), the outpatient clinic accepts walk-ins from 8 AM to midnight until the close of service on the 31st of October. The hours for some specialized services differ somewhat. Tel.: 02.5799.3913.
Costs? If you're a public health system card-carrying Italian or citizen of the European Union, there is no charge for the initial walk-in evaluation medical visit. The costs for further treatment are regulated according to your local public health system charges.
For all others, the cost for the evaulation visit is Euro 50.00, payable also by credit card. The cost for any other treatment? ASK.
Want more info? Go to this page...in Italian, sigh...to download the English language pdf. (OMG, why don't they have native speakers do the translations...there are so many in Milan. It's embarrassing to read "rates exposed," instead of "displayed," as if they were a group of flashers.)
But...it's better to stay well! Bring a sweater to Expo, you'll need it for the freezing interiors contrasting with the scorching exterior temps and sun, for which you'll need a sun hat. Bring a plastic bottle of water (glass and cans not allowed). There are two stands, one pretty much at each end of the promenade, that offer free tasty drinking water.
Enjoy! I did!