Do I need any especial vaccine?
No, you need no vaccination certificate in order to visit this beautiful destination.
What should I do in case of illness?
Most of the big hotels have doctors amongst their staff, though you can also go to any of the many hospitals and clinics in Guadalajara, installations that have an excellent infrastructure and emergency rooms as well (See Hospitals and Clinics in Guadalajara).
Is tap water reliable?
Most hotels and restaurants offer their clients purified water. Although, it is advisable to ask before you drink it. Another possibility is to drink bottled water only, thus avoiding any potential health inconveniency.
What precautions should I take regarding food?
Gastronomy in Guadalajara is usually spicy, over seasoned and greasy. Their dishes are excellent though it is recommendable to eat frugally during your first days in here. Do not eat in excess until your stomach gets used to local seasoning.
Likewise, avoid eating in places that do not show the proper hygienic conditions. Regarding the fruits, prefer those that can be peeled, and in the case of salads, ask for them at very clean tourist restaurants only. Be also prudent concerning fish and seafood. Eat them only at places you regard as reliable.
If despite all these precautions you happen to suffer a stomach problem, a very common malady amongst travellers, drink plenty of purified water and anti acids after each meal. If the problem persists, ask a doctor.