We live in a global world. Doubt any of the new immigrants have not interacted with people from various cultures and they are not coming from isolated communities. The same flu strain usually now goes around the world every year. If you have a minor infection, a cold or the flu and are not a baby or elderly/immunocompromised person there is no need to go to the ER. It is a waste of resources and time for the patient. They are also spreading a cold or flu to sick people in the ER. Many medical systems use the ER as 1st line care. We have GPs and after hours clinics.
This is where you are mistaken. We do not live in that global world as you might think. According to your theory, if there is for example big problem with some contagious illness in one region (birds flu or something else) all countries around the world will get the same amount of it.
In reality some countries may not get even a single case of it, despite opened borders. What does happen is, that the further you live from that region and the less intensive exchange of travellers you have with them, the less cases of illness you will get in your country. And few isolated cases are for sure not enough for the most of the population to build up the needed immunity there.
So no many illnesses are more specialized and more regional than you would think. (for example you will not get same type of vaccination for Borrelia - an illness transmitted by ticks; in Europe or in North America, simply because the germ causing it is a bit different in both regions).
Additionally, you can have flu with complication (for example when at the same time you are attacked by virus and by some aggressive bacteria that can cause bronchitis or pneumonia; which untreated can lead to heart problem).
There are also certain type of viruses that will cause your immune system overreact, so it will be overheating and flooding your respiration system (just check the big flu pandemic in North America in 20th century; the most affected were young healthy adults).
As for spreading illnesses - many viral infections are the most virulent (contagious) during the incubation state (just before you will get symptoms of the illness). That is usually 24-48 hours before the first symptoms and within first 24-48 hours where the symptoms are mild. After that your immune system will kick in fully so, most of the viruses that will come out of you will be the dead one in your body fluids (like cough or tears).
So your best chance to catch something is in - public transport, shopping malls, big open work space, schools, public washrooms.
Interestingly people tend to be more cautious when it comes to behaviour in ER waiting room, so the only higher risk you will have there is chance of getting something rare, that you would have in public areas.
And did you know that each years there are tens of cases of adults in Canada that will die on flu?