There are no special vaccinations required to enter Jamaica.
HEAT
Jamaica is a tropical country with temperatures rising well above 38°C in the middle of summer. Sensible precautions should be taken, especially for those not accustomed to being under such hot sun. A wide-brimmed hat is advisable for days at the beach, and a high-SPF sunblock essential. Being in the water exacerbates rather than mitigates the harmful rays, creating a risk for overexposure even while swimmers may be unaware of the sun's effects--until the evening, when it becomes impossible to lay down on a burned back. While most hotels offer air-conditioning, just as many have been constructed with cooling in mind to obviate the need for air-conditioning. Louvered windows with a fresh sea breeze or ceiling fan can be just as soothing as air-conditioning, while not putting such a strain on Jamaica's antiquated and inefficient electrical grid. In the summer months, air-conditioning is a well-appreciated luxury, especially for sleeping. If you are traveling between June and September, consider spending some time in the Blue Mountains, where there's a cool breeze year-round.
SEXUALLY TRANSMITTED DISEASES
Jamaican culture celebrates love, romance, and intimacy. While not everyone is promiscuous, keeping multiple sexual partners is common, and infidelity is generally treated as an inevitable reality by both men and women. The obvious danger in this attitude is reflected in a high incidence of STDs on the island, including underreported figures on AIDS/HIV infections. If you engage in sexual activity while in Jamaica, like anywhere else, condoms are indispensable and the best preventative measure you can take apart from abstinence.
CRIME
Unfortunately, criminal acts are a daily reality for a large number of Jamaicans, from the petty crimes committed by those who find themselves marginalized from the formal economy to high-rolling politicians and drug dons who control the flow of capital, illegal substances, and arms on the island. In sharp contrast to other developing nations with high poverty rates, and perhaps contrary to what one might expect, random armed assault on individuals and muggings in Jamaica are quite rare. The crime that is most ingrained and more or less the order of the day is devious, petty thievery. Almost everybody who has stayed in Jamaica for any length of time has experienced the disappearance of personal effects, whether a wallet or a perfume or a cell phone, one of the most prized items. Stay vigilant and take every possible precaution and you will likely have no problem.
BRIBERY
Officially bribery is illegal, and people offering a bribe to an officer of the law can be arrested and tried in court. It's generally quite obvious when a police officer is seeking a pay-off. Phrases like, "do something for me nuh," "gimme a lunch money," or "buy me a drink" typically get the message across quite effectively. Do not try to bribe police when it is not solicited (or even when it is); there are officers of the law who will take offense and could even try to use this to add to the severity of the alleged offense (or required bribe).
There is a department, the Office of Professional Responsibility (OPR) within the Jamaican police force dedicated to routing out corruption. The office is based in Kingston (tel. 876/967-1909, 876/967-4347 or 876/924-9059) but has officers across the island. Be sure to take note of the badge number of the officer in question if you are planning to make a report.
DRUGS
Jamaica has a well-deserved reputation as a marijuana haven. Contrary to what many visitors believe, marijuana is classified by the Jamaican authorities as a drug and is illegal. Practically speaking, however, marijuana use is not criminalized and it's impossible to walk through Half Way Tree in Kingston or Sam Sharpe Square in Mobay without taking a whiff of ganja, as the herb is known locally. Nevertheless, if a police officer sees a tourist smoking, it often provides a good excuse for harassment and threats of imprisonment. These are generally not-so-subtle hints that a pay-off is in order. It's not generally a good policy to entertain bribes, but some tourists caught in this situation have found that US$20 can go a long way in preventing discomfort for all parties involved.
Beyond ganja, Jamaica has also gained a well-deserved reputation as a transshipment point for cocaine originating in Colombia. Crack addiction has been a problem in some coastal communities where cargo has inadvertently washed ashore. While marijuana use is tolerated on the island due to its widespread consumption and a Rastafarian culture that incorporates its use into religious and recreational practices as a sacrament, there is no good reason to use cocaine or any other hard drug in Jamaica, despite offers that will inevitably arise on a walk along Seven-Mile Beach in Negril.