Earlier this year, Raja and Priti Narasimhan kicked off a road trip across India armed with one message: stop spitting in public. The couple carried a loudspeaker and blared their message from inside a car covered in anti-spitting slogans.

If you have spent time in India, you know what the Narasimhans are up against. Saliva graces the streets. Sometimes plain and phlegmy, sometimes blood red from chewing tobacco-laced betel nut or paan, it decorates simple walls and mighty edifices alike. It even threatens Kolkata city's historic howrah bridge.

So the Narasimhans travel the country, aiming to protect its streets and buildings and bridges from the public spitters. They live in Pune city, and have been self-appointed warriors against the spitting scourge since 2010. Workshops, online and offline campaigns, clean-up efforts with local municipalities - they have done it all. One time, Mr Narasimhan said, they painted over the paan stains on a wall at the Pune railway station only to have people start spitting on it again in three days.

"There is no reason to spit on a wall!" he says.

Reactions to their admonishes have historically ranged from indifference to anger. Mr Narasimhan recalled one man who asked him: "What is your problem? Is it your father's property?"

But the Covid-19 wave that crashed through India has changed some things, Ms Narasimhan says. Some spitters have even apologised.

"The fear of the pandemic has got them thinking," she says.

'A spitting country'

India's battle against spit on its streets has always been half-hearted. Mumbai city has tried the hardest, with voluntary "nuisance" inspectors who scold people to not spit, litter or urinate in public. But the offence of spitting has long been largely ignored.

Then came Covid, its airborne danger coupled with Indian men's love of spitting wherever they choose. Officials swung into action, penalising spitting with steeper fines and even jail time, all under the Disaster Management Act. Even Prime Minister Narendra Modi advised his countrymen not to spit in public places - something "we always knew was wrong".

This was a sharp contrast to 2016, when the health minister, replying to a question about the spitting menace, told parliament: "Sir, India is a spitting country. We spit when we are bored; we spit when we are tired; we spit when we are angry or we spit just like that. We spit anywhere and everywhere and we spit at all times and at odd hours."

He is right too. Spitting is a given on India's streets: men lounge on the side of the road, casually move their head a few inches, and let loose their saliva; men driving cars, bikes and auto-rickshaws don't hesitate to stick their heads out at traffic lights and spew. The act often comes with a warning - a uniquely guttural sound as they summon the offending sputum.

And the habit is an overwhelmingly male one. Indian men are comfortable with their bodies, says columnist Santosh Desai, "and everything that comes out of the body".

"There is an unselfish conscious ease with relieving oneself in public," he says. "If I am uncomfortable, I will immediately act up on it, the idea of restraining yourself doesn't really really exist."

Spit is also a form of "swag" that feeds into toxic masculinity, says Uddalak Mukherjee, an associate editor at Indian newspaper the Telegraph.

But why spit at all in public?

Mr Narasimhan says he has found that the reasons range from anger to "timepass" (they have nothing better to do), or simply because they can - "they feel it's their right to spit", he says.

According to the historian Mukul Kesavan, it also stems from "an Indian obsession with pollution and how to void yourself of it".

Some historians believe that this obsession can be traced to Hindu and upper-caste notions of maintaining bodily purity by discharging anything dirty outside of the home.

"Attitudes to spitting transcend questions of hygiene," Mr Mukherjee says. "A taxi driver once told me, 'I had a bad day and I wanted to eject my experience.'"

The war against spittle

It turns out, there was a time when people everywhere were spitting everywhere. In India, spitting was celebrated in royal courts, and grand spittoons were a centrepiece in many homes.

In Europe in the middle ages, you could spit during a meal, as long as it was underneath the table. Erasmus wrote that "sucking back saliva" was "unmannerly". In 1903, the British Medical Journal labelled America one of the "expectorative storm centres of the world". A Massachusetts health inspector, upon asking in 1908 why tailors spat on the floor in every factory he visited, reported receiving the reply, "Of course they spit on the floor; where do you expect them to spit, in their pockets?"

Not that things were much better in Britain, where it was common enough to spit on tram cars that people were being fined for it and the medical community demanding a law against it.

It was the spread of tuberculosis that finally dealt a blow to the habit in the West. The growing awareness of germ theory in the late 19th to early 20th Century played a crucial role, says journalist Vidya Krishnan, author of the upcoming book Phantom Plague: How Tuberculosis Shaped History.

"The awareness of how germs spread gave rise to new social habits and customs. People learned to shield their sneezes and coughs, reject handshaking, and kissing a baby was frowned upon. Domestic awareness of hygiene radiated outward as well."

Ms Krishnan says the increased awareness led to "behaviour change" in men, since they were and still are the ones "who indulge in public spitting at a scale that causes infectious diseases like TB to spread".

But India has a number of obstacles to surmount, Ms Krishnan says. Its states have never tried very hard to end the habit. And spitting is still socially acceptable - be it chewing tobacco, sportsmen spiting on camera or Bollywood portrayals of men spitting while fighting each other.

Mr Narasimhan laments the modern lack of spitoons. "Even if I have to spit, where do I spit?" he says. "As a child in Kolkata, I remember spittoons tied to lampposts filled with sand. That's disappeared, and people spit everywhere."

And there are bigger challenges. "No large-scale behaviour change or public health intervention can rule out caste, class and gender," Ms Krishnan says. "In India, access to bathrooms, running water and good plumbing are all matters of privilege."

Health experts have warned that merely punishing people, without attempting to understand why they spit, will not win the war against the habit. And two years into the Covis-19 pandemic, the zeal for curing this particular addiction is waning. But Raja and Priti Narasimhan are undeterred in their street battle. Most people remain unaware it could contribute to the spread of Covid-19, they say - and that is something they can at least change a little, if not fix.

"It's okay if we are wasting time, we will try," Mr Narasimhan says. "If we can create attitudinal change in even 2% people, then we have made a difference."