Typhoid is a major killer specially in our part of the world. 300,000 people around the world die from this illness every year and 27 million are infected. Understanding the transmission cycle of the bacteria is imperative to battle this deadly disease.

When a patient is cured of typhoid fever, there is some likelihood that she will 'carry' the typhoid bacteria, called salmonella. Salmonella in these asympotmatic 'carriers' stays in the gall bladder and intermittently with the bile which flows from the gall bladder, enters the intestines and exits in the faeces to set up a faecal-oral typhoid transmission cycle.

In a surprising finding from Patan Hospital that will soon be published in an open-access journal, PLoS One, Sabina Dongol and colleagues show that the salmonella that leaves the gall bladder in 'carriers' may not be the culprit causing the acute 'blood' infection in another person as was previously assumed. This salmonella carrier study done from 2007 to 2010 at Patan Hospital had a large sample size of 1,377 patients who required gall bladder surgery, mostly for gall stone problems. About four per cent of the bile from this group grew out salmonella from the bile culture.

The study revealed that the gall bladder salmonella and the prevalent acute typhoid fever causing salmonella appear to be two different strains as verified by microbiological and genetic studies. This mistaken identity is crucial to understand the spread of typhoid fever.

Prevailing dogma in tropical medicine states that it is the carrier who maintains this active transmission of the bacteria from person to person, but the Patan Hospital study findings counter this dogma. In a high-typhoid transmission area like ours, 'carrier' transmission seems to play a secondary role. In sharp contrast, in low typhoid transmission areas as in the Western world, it is well-established that whenever there is a typhoid outbreak, a 'carrier' is often identified and both the patient and the carrier (often a cook or a food handler) will be infected by a similar strain. So has Typhoid Mary been vindicated?

Typhoid Mary was a typhoid 'carrier' in the early twentieth century in New York who was implicated in infecting many people with typhoid in different households where she worked. Recent studies appear to question Mary's culpability in spreading this illness given that at that time New York (unlike now) was a relatively high transmission area for typhoid akin to Nepal today.

If the 'carrier' transmission is less important than was previously believed, having access to clean drinking water, using soap to wash hands and an overall improvement in sanitation (besides vaccination) will be more vital in preventing typhoid infection.