Mile End Road, coming to an abrupt stop at the intersection of Cambridge Heath and Whitechapel, is so called because it’s exactly a mile from the City of London. I know this because Herman Harcourt told me so. But then in the same breath, he also said that he’d more than once walked the entire length of Mile End Road heel-to-toe and that it had seasonal variations, contracting seven feet in the winter. For weeks he’d been alarmed about the shortening, worried that maybe his feet were still growing, before arrving at this welcome revelation. Herman considered himself something of an amateur anthropologist of the East End, but he wasn’t always a reliable witness on account of his condition.

A not-so-contestable fact is that in London’s rush hour you’ll find at least one schizophrenic on every double-decker bus – that is statistically 1:100. This was one of the first things we learned in medical school in 1981.This, along with our tutor’s mantra that there was pathology all around us, and that we should pay close attention. ‘Think pathology; always be thinking pathology!’ But how do you get from pathos to pathology? And once you arrive, is there any way back? Although I didn’t frame the question like that at the time, a version of it haunted me throughout my first year as I made my way along the Mile End Road to Whitechapel each day, either on foot, Undergound, bus or bicycle, from my shared house in Bow. There was plenty of pathology on view – TB, bronchitis, tertiary syphilis and myriad mental illnesses. Of all the schizophrenics who joined the number 25 bus on that short stretch during rush hour, Herman Harcourt was the most frequent.

The first time I came across Herman there was something familiar about him. Exiting from Whitechapel station, I heard a loud and distinctive speaky-spokey, West Indian voice. As we squeezed past the ticket collector’s box and spilled out onto the pavement, above the sing-song tirade of stallholders, discounting fruit and veg, offering ‘Pound o’ bananas fifty. Pound o’ bananas fifty’, you could just make out that singularly Caribbean accent in the distance, shouting what sounded like ‘Knee grows occurs’.

His words were high-pitched; the man appeared to be hyperventilating. The gaps between his shrieks shortened until they merged into a prolonged awful-sounding howl. At the same time, all along the pavement, one cluster after another, the pedestrians began to part. Someone or something was coming and could not be stopped. People peeled away as if instructed. It all seemed bizarre; as surreally choreographed as those feathered showgirls on television in the black-and-white musicals I’d sat through on Saturday evenings, raising their plumes in sequence to reveal, finally, the tap-dancing stars in dinner jacket and sequined evening dress.

This particular Fred Astaire turned out to be Herman Harcourt. He thrust through the mass of people on that sweltering afternoon, wrapped in a heavy overcoat with a piece of cord pulled tightly around his waist and emerged just as I knew he would, in line with the onset of déjà vu, that elusive and uncanny feeling of events unfolding marginally ahead of your memory of them.

He was no more than fifty, but old enough to have been my father, and was obviously on the run, perhaps from a shopkeeper. In each hand he gripped numerous bulging plastic bags; they formed a ring around him as encumbering as any Victorian lady’s layer of petticoats and his voice became increasingly strident and urgent until what he was actually screaming became clear: ‘Negroes are cursed!’

This strange bagman was bearing down on me and I could move neither left nor right, backwards or forwards. I was gripped by a paralysis and a collision seemed inevitable. But somehow, with his plastic bags flapping at his side, Herman brushed past, so close there could not have been a piece of tissue paper between us; so close I caught a whiff of the fug of sweat and tobacco on his dirty overcoat.

He easily outpaced his pursuers, a man and woman in their forties. If he’d continued, he might have got away, but he suddenly stopped and began walking backwards, retracing his steps precisely and nervously, as if in a minefield. When he reached me, he put down his bags, bent over and picked up a cigarette butt, pinched it back into shape and shoved it into his coat pocket. That’s when they caught up and pounced. Not in a rough way; more like anxious parents who had lost their child in the crowd. I’ve often wondered why Herman stopped. It seemed such an odd thing to do. Everything might have turned out differently had he kept on going.

‘Don’t let them take me,’ Herman pleaded. ‘Please! They’re impostors. Don’t let them take me!’

By now the man had put Herman Harcourt in an armlock. ‘Do not be alarmed, he is harmless really,’ he said. ‘No need for alarm, is there Herman?’

‘Ah who you a-call Herman?’

‘I am just saying that you are harmless, are you not? I am paying you a compliment, Herman.’

‘Listen how the man call my name. Is so you call me name, in the street? In the street?!’

‘Now Herman,’ pleaded the man.

‘I know you?’

‘Hermaaaan, pleeaasse.’

In the absence of anyone else foolish enough to slow down and pay attention to what was going on, Herman turned and confided to me: ‘Me nah know the man, you know.’

He scrutinised his captor, searching for clues, until he found that which confirmed his suspicions.

‘Look at his feet. Him an impostor. Didn’t I tell you? Him an impostor, man. Just look at his feet. You see any sock on the man feet?’

Though the man wore sensible black patent-leather shoes, Herman Harcourt was agitated and exercised over his absence of socks. I confess that I too thought it curious.

‘But Herman, you are not wearing socks either,’ said the man.

The news hit Herman hard. He trembled. Seconds passed, perhaps a minute before he steadied himself and was brave enough to look down. ‘Oh Lord dem gone with me socks!’

Herman’s anguish was pitiful, but the man seemed amused by the direction the conversation had taken. His laughing lips were held in check only by the vexed gaze of his colleague, who had a professional tenderness about her and a countenance more in keeping with a chaperone.

The setback with the socks was enough to remind Herman – how could he have forgotten? – that ‘Negroes are cursed!’ He repeated this endlessly, as if gamely trying a tongue-twister. ‘Negroes are cursed! Negroes are cursed!’ He broke free from the man’s grip. He turned and turned, not moving from the spot, like a child trying to make himself dizzy. Finally, he slumped. He couldn’t raise his suddenly heavy head; his chin rested on his chest.

The minders propped him up like cornermen to a boxer at the end of a bruising round. After some adjustment of positions (the choreography appeared to have been thought through), all three were ready to move off, not quite arm in arm but linked to such a degree that if one moved the others were bound to follow. Before they reached the corner, Herman turned back towards me. Would they allow him a word of advice to the young man? He didn’t wait for permission, but shuffled in my direction. His chaperones, loosening but not releasing their grip, snaked along with him.

‘In private, a word in private, if you please,’ he whispered.

The woman straightened the collar of Herman’s overcoat and smoothed the lapel with the back of her hand. ‘No more secrets. Remember? Who said that? Did you say it and not mean it? Was that you being insincere?’

Herman shook his head vigorously.

‘Good. So what is it you want to say?’

‘All I wanted to say to the young man . . .’ Herman began graciously, but almost immediately his tone hardened. ‘Because he looks like a decent young man, although looks can be deceiving. I wanted him to know, looking all smug and pleased with himself . . .’ He paused to jab a finger at me. ‘Brutus. Et tu, Brutus! I come not to praise him. He knows what he’s done. But I want to offer a word of advice, my friend. If they come for me in the morning, they’ll be coming for you in the afternoon.’

The chaperone squirmed as she tried to fashion some kind of apology. I thought to lessen her discomfort by saying to Herman that I had no idea what he was talking about. Except I’d met men such as Herman Harcourt throughout my young life. Yes, we Negroes were cursed, but I’d been schooled to break the spell; to confound expectations by exchanging the factory floor (which had been the lot of my parents) for medical school. That was the assumption that had been deeply invested in me. Perhaps it had once been invested in Herman too, but he had defaulted to that older, more persistent preconception of our limitations. If Negroes were cursed then men such as Herman were carriers of, and became, the virus. If he was the virus, I was the vaccine. The bagman’s presence though – his very being – mocked the notion that my contemporaries and I could escape the accursed path predetermined for the black man.

Herman’s eyes blazed with passionate intensity as he continued, with a note of compunction in his voice, ‘They’ll be coming for you in the afternoon. If not today, then tomorrow.’