I recently found that Lin, a middle school maths teacher who had just had his annual medical checkup, was worried about the "healthy" conclusion of the ensuing report.

In explaining, he broke down the situation into four possibilities: 1) An examinee in perfect health gets a good evaluation, this is the best outcome; 2) Someone in perfect health is misdiagnosed as a sick man, this is not too bad, merely wasting the money required for a recheck; 3) Someone in ill health is treated as a patient, this is also not too bad, as timely attention could help him recover or at least leave him enough time to make preparations; 4) The worst situation is the one in which a diseased person is misdiagnosed as being in perfect health.

While I was surprised at his in-depth analysis of a regular medical checkup, Lin told me that last year, one of his colleagues went through such a checkup with an apparently satisfactory result, only to die of cancer six months later. The experience had horrified Lin so that he now doubted such medical evaluations.

"I received my health report yesterday, it was perfect," he said, gravely concerned. "You know, to rule out the worst of the four situations, I have to give up the best one and prefer to suppose I have some disease."

Staring at him, I could not say a word. Mathematics is always confusing to me, and I am especially weak at probability theory.

Believe it or not, what he said did cast a shadow over me. I also had my regular medical checkup recently. Mostly this involved standing in different queues. The actual checkup only added up to about 20 minutes. According to the report, I am in sound health. Yet my stomach has been killing me these days. "If you feel uncomfortable somewhere, you may want to go to a major hospital for further checks," the doctors said, shrugging at my questions.

Several days later I met Lin again and asked whether he felt better. "I've dug out the cause of my worries," he said excitedly, adding that the recent Colgate case had given him a clue.

Lin noted that the incident happened in Britain and was directly announced by the government, but British citizens didn't seem to panic about it. In China, however, people received news through indirect channels and overreacted to it by throwing away their toothpaste. While the calm response of British people revealed their trust in a system that guarantees the safety of products for sale, a host of cases in China, such as the one involving "poisonous ham", has kept our nerves on edge. The frequent reports about fakes and inferior products have turned us into birds startled by the mere twang of a bow-string.

"Then I came to understand that my worries resulted from my distrust of our medical system," Lin said, announcing his conclusion. "My report didn't really reveal my health conditions, it might be a careless piece of work by some doctors, who knows?"

We looked at each other and decided to visit several other big hospitals for re-checks.