Greece's strong summer sun and low humidity can lead to sunburn or sunstroke if you're not careful. A hat, a light-color long-sleeve shirt, and long pants or a sarong are advised for spending a day at the beach or visiting archaeological sites. Sunglasses, a hat, and sunscreen are necessities, and be sure to drink plenty of water. Most beaches present few dangers, but keep a lookout for the occasional jellyfish and, on rocky coves, sea urchins. Should you step on one, don't break off the embedded spines, which may lead to infection, but instead remove them with heated olive oil and a needle. Food is seldom a problem, but the liberal amounts of olive oil used in Greek cooking may be indigestible for some. Tap water in Greece is fine in most urban areas, and bottled spring water is readily available. Avoid drinking tap water in many rural areas.
In greener, wetter areas, mosquitoes may be a problem. In addition to wearing insect repellent, you can burn coils ("spee-rahl") or buy plug-in devices that burn medicated tabs ("pah-steel-ya"). Hotels usually provide these. Citronella candles are usually an effective and more natural way to keep insects away. The only poisonous snakes in Greece are the adder and the sand viper, which are brown or red, with dark zigzags. The adder has a V or X behind its head, and the sand viper sports a small horn on its nose. When hiking, wear high tops and hiking socks and don't put your feet or hands in crevices without looking first. If bitten, try to slow the spread of the venom until a doctor comes. Lie still with the affected limb lower than the rest of your body. Apply a tourniquet, releasing it every few minutes, and cut the wound a bit in case the venom can bleed out. Do NOT suck on the bite. Whereas snakes like to lie in the sun, the scorpion (rare) likes cool, wet places, in woodpiles and under stones. Apply Benadryl or Phenergan to minor stings, but if you have nausea or fever, see a doctor at once.
For minor ailments, go to a local pharmacy first, where the licensed staff can make recommendations for over-the-counter drugs. Most pharmacies are closed in the evenings and on weekends, but each posts the name of the nearest pharmacy open off-hours. Most state hospitals and rural clinics won't charge you for tending to minor ailments, even if you're not an EU citizen; at most, you'll pay a minimal fee. For a doctor or dentist, check with your hotel, embassy, or the tourist police.
Do not fly within 24 hours of scuba diving.