“You’ve been a very naughty boy.” The nurse shook her head as she checked my medical records. “One, two, three, four … oh, and five. Five vaccines overdue.” Shame-faced, I stared down at my fidgeting hands. I was 44 years old.

“You should have had your hepatitis B booster a few years ago, but you’re probably still okay with that one. That leaves tetanus, diphtheria, typhoid and yellow fever. Roll up your sleeves. We’ll do two in each arm.”

The nurse jabbed the needles into my skin without sympathy. I gritted my teeth as the prescribed doses were injected. I would feel ill for the next two days, but my odds of picking up a nasty disease during my forthcoming trip to the Amazon rainforest had been considerably reduced.

Thanks to vaccines, the world is open to modern travelers. We can contemplate taking holidays in tropical Africa which, due to the virulence of local diseases, was formerly known as the “white man’s grave.” We can sail through the Panama Canal where thousands of workers died of yellow fever and malaria during the initial construction. We can tramp through the Amazon rainforest, immune to most of the health hazards we might be exposed to. And we can interact with indigenous people without fear of passing on common diseases, such as mumps or measles, to which they have no natural immunity.

I am living proof vaccines work. During my childhood in East Africa, I was exposed to numerous parasitic diseases and viruses that only a generation earlier would probably have killed me. When I contracted malaria, the symptoms were diluted thanks to the pills I took before, during and after. And the mystery fevers that occasionally laid me low for a day or two may have been the suppressed symptoms of typhoid, cholera or diphtheria.

Vaccines have been such a routine part of my life I have taken them for granted. Which was why I found myself exposed to the withering ire of a nurse. I hadn’t avoided keeping up to date; I’d merely become so blasé about the process I’d forgotten to check.

It was a reminder you should never drop your guard concerning travel health. The threats in different parts of the world are constantly changing. Epidemics flare up. Diseases develop localized drug resistance. Some diseases are wiped out in certain countries only to reoccur suddenly in others.

Meanwhile, the arsenal of weapons to combat diseases is also ever-changing. New vaccines are being developed while the performance of existing prophylaxes is always under scrutiny.

When I first received a course of vaccines against hepatitis B, the official medical advice was that a booster injection would be required every five years. Current research now suggests the primary course will provide indefinite protection — which was why the nurse wasn’t concerned about that particular omission in my records.

By contrast, the daily pills I used to take to prevent malaria are now completely ineffective. In the past decade, as more virulent strains of the disease emerged, choosing the right malaria protection has become much more complicated, with a range of preventative treatments available depending on the individual and the countries he or she intends to visit.

Prevention of malaria, which is carried by mosquitoes, has become more complicated over the years. © Dmitry Knorre | Dreamstime.com

The website of the Centers for Disease Control and Prevention provides the very latest health advice for individual countries, as well as plenty of useful tips about staying healthy while traveling.

However, every individual is different, and sometimes our personal medical history dictates the specific treatments we should receive prior to traveling. Therefore, it is essential to consult your doctor before any major trip, especially if you are traveling to the Third World. In addition to ensuring all of your vaccines are up to date, you may also be advised to travel with a medical kit tailored to your individual requirements.

In general, the onus is on travelers to ensure they are appropriately vaccinated for their destination (though failure to be fully protected may have adverse implications for your medical insurance). However, some countries require you to produce a yellow fever vaccination certificate on arrival if you are traveling from — or have even transited in — a country in which the disease is present.

It is difficult to imagine what the world was like before vaccines became commonplace. Every time an insect bit you, every time you ventured into a crowded place, every time you took a drink or ate a meal, you risked contracting a fatal illness.

One of the nastiest viral diseases provided the key to immunization. Throughout human history, communities were haunted by the specter of smallpox. On average, 30 percent of people infected would die, while many others would be maimed for life. With some strains of the disease, fatality rates were greater than 90 percent. There appeared to be no defense against this stealthy killer, which could appear at any time in any place.

Smallpox was indiscriminate, infecting royalty and paupers alike. However, in the late 18th century, English physician Edward Jenner was intrigued by common hearsay that farm workers were immune to smallpox if they had previously been infected with cowpox, a related but much less serious disease.

Jenner speculated that if people were deliberately infected with cowpox, they would acquire smallpox immunity. To test his theory, he needed a human guinea pig. On May 14, 1796, James Phipps, the 8-year-old son of Jenner’s gardener, became the first recipient of a vaccination.

Live cowpox was injected into the boy’s arm. Phipps fell ill for a couple of days but then recovered. On July 1, Jenner made a giant leap of medical faith when he rolled up the boy’s sleeve and injected smallpox into his arm. Phipps developed no symptoms. A preventative treatment against smallpox had been discovered.

Although there had been previous attempts elsewhere at devising forms of inoculation, Jenner is widely credited as being the originator of modern vaccination. Following his breakthrough, vaccines for other diseases were developed, and as governments began to introduce programs for mass vaccination, scientists raised the possibility that some diseases could be completely eradicated.

Again, smallpox led the way. In the 20th century, it is estimated the disease claimed up to 500 million lives. From the 1950s onward, coordinated efforts were made to vaccinate millions of people across the world against smallpox. As the percentage of people with immunity rose, so the number of cases plummeted. In 1980, the World Health Assembly confirmed smallpox had been eradicated (though small batches of the virus were retained in laboratories for research).

Not everyone is in favor of mass vaccination programs. Ever since Jenner’s breakthrough, a vocal minority has opposed the practice on political, medical or religious grounds.

The political argument tends to center on the rights of the individual. Compulsory vaccination programs need to reach the majority of a population in order to be fully effective, but to what extent should individuals have the right to opt out? Every U.S. state permits exemptions in certain circumstances (primarily people who have religious or philosophical objections).

The medical argument is more complex. Some experts have warned about the potential for vaccine overload, which may compromise the immune systems of some individuals and reduce resistance to diseases among the wider population.

Others raised specific concerns about particular vaccines. A high-profile campaign in the 1990s claimed a direct link between the MMR (measles, mumps and rubella) vaccine and the global rise in autism. No link has ever been established, but the temporary fall in MMR uptake in some countries has been blamed for recent outbreaks of measles.

Vaccinations often lead to temporary side effects. Most of us have experienced aching arms or a mild fever following an injection. In some individuals, the reaction is more extreme. In 2011, BBC reporter Malcolm Brabant suffered a mental breakdown he is convinced was caused by a yellow fever vaccine. He has written about his experiences in a compelling book, Malcolm Is a Little Unwell.

The religious argument tends to regard vaccination as being contrary to God’s natural law. Perhaps the most radical expression of that position has been in Pakistan, one of the three remaining countries (the others are Afghanistan and Nigeria) in which polio still occurs. Several health workers engaged in the polio vaccination program in rural areas have been murdered in recent months by Pakistani religious extremists, whose increasingly violent campaign is one of the final obstructions to the complete global eradication of polio.

The development of vaccines helped to facilitate mass international travel. Ironically, the huge volume of people now traveling throughout the world is potentially a major health threat for the future. When new diseases emerge, they can rapidly infiltrate all corners of the globe, spreading unchecked until suitable vaccines are developed to thwart them.

We can’t protect ourselves against the unknown. But with regard to existing vaccines, I’ve learned my lesson. I regularly check to ensure everything is current. I’m scared of traveling without protection against the known diseases that lurk around the world. And I’m absolutely terrified of the nurse at my local travel clinic.

To keep up to date with the latest information on which vaccines are recommended for each country and where to get them, visit the Centers for Disease Control and Prevention website.