Infectious diseases have been a part of the human condition since time immemorial. Some, such as mumps or chicken pox, usually have mild symptoms and mostly lead to recovery. Others, such as HIV, tuberculosis or malaria, are responsible for millions of deaths each year. The 1918 Spanish influenza pandemic killed 50–100 million people in the space of six months.
Sociological upheavals following the black plague led to the demise of the church as an all-powerful institution[ii], the destruction of the serf system and subsequent creation of labour movements and colonialism[iii]. The universal symbol for a medic — the Rod of Asclepius — is an image of worms wrapped around a stick, because that was the only way to remove Guinea worms from the body, so having a stick meant you were a doctor[iv]. Malaria was one of four diseases that were targeted for eradication in the 20th century; the failure to do so led directly to the creation of the environmental movement..
Despite its massive death toll, smallpox remains the only successfully eradicated human disease (but not for long!), thanks in part to a successful vaccine, in part to photographic-recognition cards and in part due to forced vaccinations of whole villages in the former Yugoslavia[vii]. This brings up issues of individual rights versus public good that remain relevant today. Other momentous achievements are the virtual disappearance of erstwhile disabling and lethal diseases such as diphtheria, tetanus, paralytic poliomyelitis, pertussis, measles, mumps, rubella and invasive H. influenzae b.
Diseases have been the source of fear and superstition throughout the ages and continue to pose a threat, from the irritation of the cold and flu season to the potential extinction of our species. But what are the worst of the worst? In order to understand what that means, we need a measure of what constitutes “bad”. A disease that doesn’t kill but leaves you disfigured and unable to work (as elephantitis does) inflicts enormous suffering, both personally and economically. Those things are hugely important, but they’re not the focus of this book.
Here, we will centre on the biggest, baddest diseases of all time using the sheer number of deaths as the measure of damage. Largely, that’s because this is an easily definable metric, although it ignores the relative sizes of population. The Mexican haemorrhagic fevers of the 16th century might have reduced the population by 28 million, a lot less than Spanish influenza’s 50–100 million deaths... except that the Mexican population was only 30 million when it started, making for a devastating 93% death toll, whereas the 20th century’s largest pandemic “only” killed 5.5% of the world; this would be equivalent to 350 million deaths today.