How many times do you wash your hands?

Johnson and I are among the millions battling obsessive-compulsive disorder, an anxiety disorder characterised by intrusive distressing thoughts and repetitive rituals aimed at dislodging those thoughts. We are an eclectic bunch spanning every imaginable cross-section of society and we battle an equally eclectic mix of obsessions and compulsions. Some of us obsess about contamination, others about hurting people, and still others about symmetry. Almost all of us can find something to obsess about at a restaurant.

Many of the situations that unsettle people with obsessive-compulsive disorder — driving, for instance — provoke at least some level of anxiety in just about everyone.

But restaurants are designed to be calming and relaxing. But we walk into a calm and civilised dining room and see things we won’t be able to control. This feeds directly into one of the unifying themes of the disorder: an often-crushing inability to handle the unknown.

“The common thread, I think, has something to do with certainty,” said Dr Michael Jenike, medical director of the Obsessive Compulsive Disorders Institute at McLean Hospital in Belmont, Massachussets, affiliated with Harvard Medical School.

“If you have OCD, whatever form, there seems to be some problem with being certain about things — whether they’re safe or whether they’ve been done right.”

If lack of certainty is our common challenge, than warding off uncertainty is our common quest. For some of us battling OCD, that means scrubbing our hands to make sure they’re clean, or checking and re-checking everything around us in the name of safety. For others, the need is to arrange various items in order, or repeat actions in ritualised sequences in vain attempts at removing doubt. In a restaurant, once Johnson gets past the door, she often needs to try out a few tables, looking for one that feels right, as a frustrated maître d’ looks on.

Personally, I am fine with tables. But I have harm obsessions, which means I am plagued by the fear that other people will be hurt by something I do, or don’t do. Seated at a less-than-sturdy table, I conjure images of fellow diners being injured should I fail to notify the management. This is called a “reporting compulsion” and before I learned to fight these urges, many a manager heard from me.

It’s what’s on top of the table, and precisely where, that really matters, says my friend Matt Solomon, a 39-year-old lawyer in Fort Worth with order compulsions. To enjoy a meal he needs to separate the salt and pepper shakers and ideally, place a napkin holder or other divider midway between them.

Why? He can no more answer that than Johnson can tell you why she needs to chew her food in sets of three bites or drink her beverages three sips at a time. Three is her magic number. That is about as refined an explanation as any of us can give for our compulsions, rituals that we understand are entirely illogical.

And waiters, waitresses and bartenders. For some with obsessive-compulsive disorder, the success or failure of a dining experience can hinge on the appearance of a restaurant’s staff.

Solomon, for example, feels compelled to inspect the hands of anyone serving him. Cuts and scrapes are objectionable because in his mind, they can lead to his contracting a disease that could kill him.

Recently, Solomon ate at the bar where he noticed a small red stain on the bartender’s right knuckle, and thought it was blood. Trying to avoid things the bartender had touched, he used a straw to drink water and swapped the silverware.

Jared Kant, a 24-year-old research assistant in Boston who has obsessive fears of contamination, once insisted on sterilising all utensils at a Chinese restaurant table by holding them above a small flame at the center of a pu-pu platter.

Coincidentally, Solomon and Kant have each battled contamination issues on both sides of the counter. Solomon spent years working as a bartender, often consumed by thoughts of becoming deathly ill. He was convinced that one of his regular customers was carrying a fatal virus, and came up with strategies to minimize contact. “I would always quickly put his change down before he could try to take it from my hand,” he said.

The challenge for Kant was serving lattes. In his late teens, while training to be a barista, he learned of the potential dangers from improperly handled milk. He became obsessed with the possibility of harming customers through inadvertent negligence. Even worse was the prospect that he might never know.

As part of my harm obsession, one of my concerns is that germs from my mouth will hurt others. Although I try to keep my fingers away from my lips and their germs while I’m eating, I’m rarely successful. By the end of the meal, I believe that my hands are contaminated. The problem is that I need them to scribble my signature on the check.

If I’m lucky, I will have remembered to bring my own pen; if not, I may feel compelled to wash my hands. Forget drying them, my napkin would only re-contaminate them.

Once the check is signed, I must be sure that it is really signed. At my worst, I have opened and closed the vinyl check holder again and again, seeing my signature each time, yet unable to feel certain. I’ve left the table, only to return to check again. And again.

Help is available, in the form of a therapy called exposure response prevention. The technique calls for exposing people with obsessive-compulsive disorder to situations that trigger obsessions, then preventing them from acting on them. The therapy addresses low-level anxieties, and works up from there.

With restaurant cleanliness, for example, a therapist might have a client rate his anxiety about challenges ranging from simply touching spotted silverware to eating from a spotted plate. Then the therapist would ask him to face those situations while fighting the compulsion to clean or replace spotted items.

The therapy attempts to alter behaviour, but it appears to alter much more than that. Dr Sanjaya Saxena, the director of a programme for obsessive-compulsive disorders at the University of California at San Diego, said that exposure response prevention therapy “certainly is changing the brain at the molecular level — that is, at the level of particular proteins that are expressed and created and on the level of neurotransmitter function.” In that sense, he said, “behavioural therapy is biological therapy.”

I am no brain scientist. I understand almost nothing about proteins and neurotransmitters. But my own work with this treatment has progressively allowed me to take back much of the life my disorder stole from me. Now when I say check, please, I’m simply asking for my bill.