Special-interest tourism is a fast-growing and highly lucrative alternative to traditional mass-market travel, although if you find yourself in Mecca during the haj, you might fail to see the distinction.
Pigeonholing holidaymakers as mainstream or niche is no easy task: does taking a tour of St Mark's Basilica in Venice, or knocking back caipirinhas at the Rio Carnival make you a religious tourist?
As motives for travel diversify, so do the number of subsectors. Health tourism can refer to an affordable operation in a faraway hospital but it also encompasses a weekend of meditation and yoga at a spiritual retreat.
Culinary tourism could mean a visit to a local market to choose ingredients for a cooking lesson but, if you stop at a vineyard en route, have you become a wine tourist?
Stay overnight at the vintner's farmhouse and by some definitions you're now an agritourist as well.
Here are six special-interest genres enjoying unprecedented growth.
Health and wellness holidays are outpacing other forms of niche travel as a result of increased affluence, ageing populations and the rise of low-cost airlines.
Valued at between US$40 billion ($55.1b) and US$60b in 2017, medical tourism is a diverse field that pairs an uninsured United States citizen needing knee surgery with a private hospital in Ecuador or a Canadian with a cataract specialist in Cuba.
A Briton will save a small fortune (and avoid long waiting lists) by having titanium dental implants fitted in Budapest, Hungary, while savvy Hongkongers head to Manila, in the Philippines, for the same procedure.
On the downside, follow-up consultations can be difficult to arrange if you had your triple heart bypass in, say, Brazil.
Needless to say, scare stories abound and, when operations are botched, patients often return, tail between their legs (where it was accidentally grafted), to have things put right back home.
Expect an increase in the number of TripAdvisor-style medical peer review websites rating surgeons and the overall hospital experience.
And after your op, what better way to get back in shape than to book a cycling holiday or a ski trip?
Sports tourism is another huge money-spinning market segment and ranges from overseas school rugby trips to a golfing weekend in Chiang Mai, Thailand.
The trend is a win-win for governments, both as an engine for economic growth and from a public-fitness perspective. Sports lovers heading abroad to compete in half marathons or five-a-side football tournaments can only be beneficial for the collective health of a nation, although environmentalists have concerns.
Besides participants, the carbon footprint of spectators who travel to watch their favourite teams can be colossal.
Three Uefa Champions League finals in the past five years have involved clubs from the same country having to trek across Europe to a stadium in another.
In 2016, for example, 39,000 fans from Madrid, Spain, did their bit to accelerate global warming by making the 3,100km round trip to Milan, Italy, to watch their sides - Atletico and Real - play each other.
Death, disaster and doom draw ghoulish sightseers to all sorts of sinister places.
Curious travellers can don a radiation suit and explore the abandoned Ukrainian city of Pripyat, near the Chernobyl nuclear power plant, which blew its top in 1986, or ponder the atrocities committed at the My Lai massacre site, in Vietnam.
Despite the intervening two millennia, the ruins of the Roman town of Pompeii, which was buried after the eruption of Mount Vesuvius in AD79, remain as chilling and macabre as Tuol Sleng, the Cambodian high school used as a torture chamber by the Khmer Rouge in the 1970s.
Dark tourists travel far and wide to sites of suffering - not everyone folding paper cranes at the Hiroshima Peace Memorial Park is Japanese nor are all visitors to New York's National September 11 Memorial and Museum (Ground Zero) American.
Perhaps the most ancient special-interest holiday sector of the lot, religious tourism refers to travel that focuses on faith-based activities and traditions; art, architecture and culture.
The Muslim haj to Mecca and Camino de Santiago, a four-to six-week Christian pilgrimage through France and northern Spain, are celebrated manifestations of the genre but experiencing the Black Nazarene procession in Manila or a wild Mardi Gras weekend in New Orleans, in the US, are also revenue generating examples.
Best of all for dedicated tour operators, holy holidays tend to be much less affected by travel trends, economic upheaval or terrorism than more conventional forms of tourism.
A force for good in an unequal world or a compulsory hoop for gap-year students to jump through?
Community-based tourism could involve a night at a homestay in a South African township or refreshments in a favela cafe perched high on the hills above Rio de Janeiro, Brazil.
It might mean an afternoon nosing around the labyrinthine alleys of Dharavi, in Mumbai, India, on a Slumdog Millionaire tour. Whether the excursions offer a genuine insight into the workings of a marginalised section of society or merely provide an opportunity to gawp at less fortunate souls for a few hours depends on your point of view.
Detractors argue that many tourists are motivated by curiosity bordering on voyeurism rather than compassion.
Community-based tourism's slightly imperialistic sibling, Voluntourism, sees well-meaning Western teens pay for the privilege of digging ditches or building (wonky) schools that villagers sometimes have to pull down and reconstruct once the youngsters have moved on.
In fairness, much good work is done by armies of adolescent foreign labour but surely the locals are capable of digging their own wells? Cynics have long suggested that the gap-year traveller's resume is the real winner.
Celebration tourism and enotourism
Stag and hen parties are another area enjoying significant growth, despite a reputation for raucousness that has resulted in attempts by some European cities to ban the boozy bashes completely.
At the less raucous end of the scale, birthday and anniversary breaks are another buoyant segment of the market; as is enotourism - the tasting and purchase of wine, usually at the site where it is produced.
An estimated 10 million tourists sampled the delights of French vineyards in 2016, attending workshops, cycling wine routes and staying overnight in vineyard accommodation.
So there you have it. Head to Prague, in the Czech Republic, and drink copious quantities of beer to celebrate a final night of being single and authorities throw the book at you. Get sloshed on similarly excessive amounts of wine at a cellar tasting in Provence and everyone's happy.
- South China Morning Post