How can one prevent heat strokes in the summer due to high temperatures, and how can they be treated in first aid?
For starters, make sure you or someone close by is calling for an ambulance, because you need that more advanced help started in your direction ASAP.
Get the person out of the heat, into air conditioning or at least out of the sun. If there are no air conditioned areas close by then shade is next best. Get them undressed if possible, soak them with cold water. If you're alone, you may not want to just place them in a cold bath, covering them with sheets soaked in cold water would be helpful. If they are still conscious and able to tolerate it, you may place them in a cool bath or shower. If they are still conscious and able to safely swallow, you may give them cold water to sip (not gulp, you don't want them vomiting).
Let's discuss what heat stroke is... Signs and symptoms of heatstroke as presented by the Mayo Clinic include a rectal temperature of 104 degrees Fahrenheit or higher, altered mental status, nausea, decreased sweating and rapid heart rate.
For a complete list of signs and symptoms, see this link from The Mayo Clinic.
For diagnosis and treatment information for heatstroke, visit this link from The Mayo Clinic.
If you would like an in-depth read on the history of heatstroke, risk factors, causes, in-hospital treatment of it and a variety of additional information on it, see this link from Lippincott Nursing Center. The link goes to a continuing education article on heat stroke for nurses. In my personal opinion, this is the most informational article of the three I shared. However, it is wordy and a rather lengthy read.
Hope this helps!