Dengue fever (not to be confused with yellow fever) is a disease transmitted by mosquitoes, that's present in Costa Rica. There is no known vaccination for dengue fever at the present, only prevention and immediate hospital treatment for those infected.
Infection rates reached an all time low in 2014 and seem to be maintaining that level. Still, health services report between 20 and 30 cases every week, so there's no reason to let your guard down. The use of mosquito repellent should be maintained, especially on the coasts.
Symptoms of dengue are similar to the flu or food poisoning, dengue can evolve to a more severe variety if untreated, especially in people who have been previously infected. The more severe hemorragic variety can cause permanent damage to organs, and even death.
The disease starts with a high fever, over 40 C (104 F), 5 to 8 days after the mosquito bite. The fever is accompanied by upset stomach, headache (usually described as felt "behind the eyes"), and joint and bone pain. A rash may also be present, similar to the rash that occurs with measles. After the fever goes down, swelling and some bleeding may occur. Recovery normally takes about 10 days, sometimes more.
There's 3 types of dengue fever active in Costa Rica at the present. Getting the disease gives you immunity, but only to the type you were infected with. Complications arise quickly if you're infected with a new type, so it's not a viable option to become immune by being exposed.
To prevent infection, always carry and wear a deet-based insect repellent. Transmitting mosquitoes can be found everywhere in the country, although they're more abundant on the Atlantic and central Pacific coasts. To reproduce, they need clean standing water, which is easier to find in these areas and usually isn't covered by fumigation, applied in urban centers.
Apply repellent on all areas of your body not permanently covered. This incluyes your feet, shoulders, back (commonly overlooked), and back of your hands. Spray your clothes for added protection. If you find a mosquito bite on you, take it as a warning: you're either not applying the repellent correctly, or you're using one which is not strong enough.
At night, keep mosquitoes out of your room using repellant heaters. These small dispensers plug into a 110V outlet and carry a small chemical plate inside of them, when they heat up, they release mosquito repellant into the air for 8 hours or so. You can find them at most supermarkets. To keep infants safe, get a mosquito netting, and place it over the crib at night.
On a side note, keep your stuff free of standing water, unless you want a mosquito breeding ground outside your room. Turn pots, pans, or anything that might collect water over, to dump out standing water. Transmitting mosquitoes usually have a living radius of 200 meters around their breeding point, so if you clear breeding points out of your living space, you're giving yourself a safe zone to be in.
If you or anyone in your group develops the symptoms described above, get them to a doctor or clinic as soon as possible. You should not let more than 24 hours pass before you seek help. Keep the person hydrated using clean, chlorinated water, or sterile fluids. Use over the counter analgesics for the pain and fever, but don't give them aspirin: aspirin is a blood thinner and can make internal bleeding worse if the person should develop it.
Blood tests exist that can identify if you have the disease, but they're used mainly as a late confirmation. These tests are conclusive only after the main infection has set in (around day 6). Before that, they won't provide much information, and it will be unclear whether you have a particularly strong flu or dengue fever. For this reason, it's best to seek help as soon as possible once you see the first warning signs.
Chikungunya is a disease that's also spreading in Costa Rica, similar to dengue. It's also transmitted by mosquitoes, and has similar symptoms. Outbreaks started in 2014, and were moderate in comparison to dengue.