Pathogens

Your skin is covered in microorganisms right now, and almost none of them are trying to hurt you. Only a small minority cause disease, and those are the ones we call pathogens. Getting that distinction right is the first mark in a lot of questions.

What counts as a pathogen

Start with the definition, because the examiner will want it in your words.

Notice the word causes. A microorganism sitting on your arm doing nothing is not a pathogen. It only earns the label if it can make a host ill.

Every disease is linked to a set of signs and symptoms. Those two words are not the same thing, and it is worth keeping them apart:

• A sign is something someone else can measure or see — a temperature of 39°C, a rash, a low blood count.
• A symptom is something only the patient can report — feeling sick, aching, being tired.

The four groups

Pathogens are not one type of organism. They are scattered across four very different groups, and the differences between those groups explain why we treat the diseases differently later on.

Communicable and non–communicable

Pathogens cause communicable diseases. That word simply means the disease can be passed from an infected organism to a healthy one, which is why you will also see them called infectious or transmissible.

Diseases that are not caused by a pathogen cannot be passed on. Those are non–communicable.

Careful observation changed medicine

Neither of the two people below could see a bacterium. Both worked out how a disease was spreading anyway, just by watching patterns closely. That is the point the syllabus wants you to take away: observation on its own can produce a breakthrough.

John Snow and cholera

Cholera is caused by the bacterium Vibrio cholerae, and it spreads through water and food. An infected person passes huge numbers of the bacteria in their faeces. If that waste reaches the drinking water, or if infected hands touch food or cooking utensils, the bacteria reach new people. This is why cholera hits hardest where sanitation and clean water are poor.

In 1854 an outbreak in Soho, London killed over 500 people in a month. John Snow, a local doctor, had already met cholera in a mining village years earlier, so he knew its symptoms and suspected how it moved. He mapped every death.

The pump handle was removed and the outbreak came to an end. Later it turned out the pump stood only a few feet from a cesspit contaminated with Vibrio cholerae. Snow’s observations helped control that outbreak and fed into germ theory, the idea that microorganisms cause disease, which transformed sanitation across the 19th century.

Ignaz Semmelweis and childbed fever

Puerperal fever, better known as childbed fever, is a bacterial infection of the female reproductive tract after childbirth. It spreads by direct contact during delivery, and in the 19th century it was the biggest cause of death linked to childbirth.

Semmelweis, a Hungarian physician, noticed one maternity ward had far more deaths than another. Watching more closely, he saw that the obstetricians and medical students on the deadlier ward went straight from carrying out autopsies on women who had died of the fever to delivering babies, without washing their hands.

He found a correlation between the number of autopsies performed and the number of deaths on the ward, and suggested that some kind of particle was being carried from the corpses to the mothers. He made handwashing compulsory, and later had the instruments washed too. Deaths fell sharply.

Worked examples