It was late afternoon when I walked in to rescue my intern from a heated conversation with a man demanding antibiotics.

“Lifelong antibiotics shouldn’t be taken lightly,” the intern implored. The man countered: “When my friend lost his pancreas in an accident the doctors said he could get meningitis if he didn’t. You want me to die?”

“Your friend must have lost his spleen”, I said, stepping in. He looked at me with narrowed eyes.

“Spleen, pancreas, isn’t that the same thing?”

Placing a hand on my harrowed intern’s shoulder, I said: “No, they are different organs. You need antibiotics when you lose your spleen.” Later, when the mollified patient had left – minus antibiotics – the intern mused: “It’s incredible he doesn’t even realise they are separate organs.”

But it wasn’t incredible at all, I replied. That morning, another patient had reported being diagnosed with polyps in two places, the uterus and colon. Months later, some bleeding had caused her to become concerned about having cancer.

“I thought some of your polyps were removed. Where were they from?” I asked.

She looked nonplussed. Her husband took out a clinical photograph: “The doctor said these ones started with ‘b’ and not to worry.” Pointing to the report, I explained: “It says these benign polyps were found at your colonoscopy, so that means it’s the polyps in the uterus that are bleeding.”

There was silence before the mother-of-three asked, in all innocence: “What’s a uterus?”

I thought of these patients when I read of a poll conducted recently by the UK cancer charity, Eve Appeal, which found that one in five women mistakenly believed that gynaecological cancer was caused by promiscuity. Women in their 40s and 50s were most likely to ignore or underestimate the significance of symptoms such as persistent bloating or a change in their menstrual pattern that can be a harbinger of gynaecological cancer – which affects 20,000 women a year in the UK.

But amid the usual (and understandable) suspects of embarrassment and stigma and dismissive doctors as contributors to a delay in cancer diagnosis, what’s rarely mentioned is the widespread health illiteracy that compromises not just cancer treatment, but indeed every aspect of healthcare.

Being health literate lies at the heart of better health. It means being able to process basic health information and use it to navigate decisions. Even more simply, it means understanding how to take a pill, read an appointment slip, and follow the signs to x-ray. It means being able to calculate how much morphine to take if you need 5mg and the bottle contains 1mg/ml. Or knowing how much insulin remains in your pen if you have injected 76 units out of 100.

But as medical advances reward us with ever more sophisticated drugs and interventions, there is little to suggest that patients are any more engaged about what it means for them. Evidence abounds as to how little our patients understand of what we tell them. Many don’t know what it means to take pills on an empty stomach or have a fasting cholesterol test.

Women who are not sexually active think that they don’t need a mammogram, and that the older they get, the lower their risk of developing cancer.

Men at risk of prostate cancer confuse a digital rectal exam with a sigmoidoscopy. This week a patient asked me if antihypertensives adjusted blood pressure up or down. Another couldn’t understand how lungs are related to the heart. A third asked if a person’s genes were stored in an organ. The list is as endless as it’s dismaying; it would beggar belief if I couldn’t attest to it.

When it comes to cancer, health illiteracy joins fear and myth to create a potent brew of misunderstanding. Words like polyp, tumour and lesion mean nothing and the same goes for terminal and malignant. Unsurprisingly, “vomit” is understood by 98% of people, but “orally” by a mere third.

“Informed consent” makes a mockery of the term. Typically written in college-level English, these forms are unintelligible by the vast majority of patients. Bureaucratic demands mean consent forms are becoming longer and more detailed but it’s rare to meet a patient who understands their content. One study reported that half of older women could not answer correctly how many times out of 1,000 a flipped coin would be likely to land on its head: common answers included 25, 50 and 250. Yet we expect these women to make informed decisions about surgery for their breast cancer and understand the risk-benefit ratio of chemotherapy. In another study, nearly half of all American adults could not perform a two-step numerical task. No wonder then that the majority of patients, men and women can overestimate their benefit of chemotherapy by 20% or more.

Health illiteracy is associated with poor surveillance, delayed diagnosis, problematic compliance and worse outcomes. Health-illiterate patients ask fewer questions, visit doctors less, and rely more on anecdotes and myths. These patients are poor at self-care and they make for poor carers. Culturally and linguistically diverse groups fare worse in every way.

Health education must start in school. We need to teach children the importance of understanding how the body works. When a woman is health literate the whole family benefits. But simply producing more material won’t do if it’s not well thought out or presented in a way that fails to engage diverse groups.

Manipulating the human genome to cure disease is the holy grail of modern medicine: the New England Journal of Medicine recently reported that last year the total support given by the US National Institutes of Health to research projects that included the word “gene” was 50% greater than funding for areas mentioning the word “prevention”. What a pyrrhic victory it would be if the health-illiterate patient is still wondering where in the human body the genes dwell, but signs the consent form anyway.