Flu viruses come in a variety of forms, some deadly, others mild, some readily transmitted, others hard to catch. The H5N1 variant, endemic in Asia since 2003, has killed over half of the 688 people who have caught it so far. These have been mainly poultry farmers, accidently contaminated by blood while slaughtering their birds. However, in 2012 Dutch scientists showed that H5N1 could become highly transmissible in mammals if just five out of the 13,000 base pairs in its genome were changed. The draconian response to what turned out to be a mild swine flu outbreak in 2009 will be repeated whenever new, possibly virulent, flu viruses are detected.
The human immunodeficiency virus was identified in 1983, seven years after ebola. It has killed 35 million people globally, and today infects 35 million others. Transmitted through exchange of body fluids, it invades, then destroys, cells in our immune system, leaving us vulnerable to a gamut of other infectious agents. A remarkable push to develop drugs has given us treatments that suppress, but do not eliminate, HIV.
In 2002 a highly contagious and fatal infectious new disease emerged. Severe acute respiratory syndrome is caused by a virus, believed to have jumped between civet cats and human beings at Chinese live food markets. The virus rapidly reached Hong Kong and from there jet planes carried it around the globe through people travelling before their symptoms had emerged. More than 8,000 people were infected and nearly 800 died. Identifying and isolating patients became key to halting the spread of the disease.
Tuberculosis, caused by Mycobacterium bacteria, has been a scourge of mankind for hundreds of years. The germs are spread in inhaled droplets spluttered from the lungs of afflicted patients. The discovery of the antibiotic streptomycin in the late 1940s brought a turnaround in our battle against TB – at least in the west (many millions remained infected in the developing world). The standard treatment lasts for many months, and using inadequate doses of drugs encourages the survival of bacteria that are resistant to the drugs we have. Extensively resistant tuberculosis (XTB) involves bacteria resistant to nearly all of the available drugs. It has proved extremely difficult to find new drugs against tuberculosis and there is a risk that the untreatable form of the disease will take a foothold globally.
This is caused by parasitic trypanosomes transmitted by biting insects, known as “kissing bugs” because they suck blood from our lips and other mucous membranes. There are about eight million people carrying the parasites. An acute phase spanning a few months of headache, fever and malaise precedes a chronic condition that can lead to death decades after the initial infection. Inflammatory responses to the parasites destroy our own organs. Transmission is confined to the Americas, where the kissing bugs live. However, as human beings migrate, they carry their parasites with them; hundreds of thousands of cases of Chagas disease are found around the world. Blood transfusions can transfer the parasite, too, and screening at blood banks has become mandatory in several countries.
Variant Creutzfeldt-Jakob disease (vCJD)
CJD is not caused by a classical infectious agent. Instead, mutant versions of proteins found in our nervous system misfold, rendering them useless. Misfolded versions can bind to properly folded versions and cause them to misfold, too. This causes large plaques to form in the brain, leading to neurological breakdown. A form of the disease, called kuru, was common among cannibals in Papua New Guinea. Shocking feeding practices that led to the ingestion of misfolded sheep proteins by cattle sparked the 1980s BSE outbreak in the UK. Many of us consumed misfolded cow prions at this time. Fewer than 200 people have developed vCJD, primarily because in human beings little cow prion protein escapes the intestine. It could easily have been otherwise.
Smallpox is the only human infectious disease that we have managed to defeat; by 1977 a global vaccination campaign had eliminated it. But vials of the causative virus keep popping up, most recently at the old National Institutes of Health laboratories in Maryland in July. Smallpox has also been a favourite bioweapon. British troops colonising the Americas gave smallpox-impregnated blankets to Native Americans; the virus then ravaged the non-immune population. US and Russian troops continued to be vaccinated against the disease throughout the cold war, each side claiming it feared a biological attack from the other. George W Bush and many US health workers were vaccinated in 2002 because of fears that terrorists might be holding the virus. Today, with modern DNA synthesis so readily available, some worry that the smallpox genome could be re-created easily, so it would not help even if all physical stocks of the virus were destroyed. How absurd if the one disease mankind had managed to eliminate came back in such a form.
Our profligate and imprudent use of antibiotics has led to the emergence and spread of bacteria that resist their effect. MRSA, XTB and a gamut of other resistant infections are widespread and kill many thousands each year. By and large the pharmaceutical industry has abandoned the search for antibacterial agents, and so things can only get worse.
Anthrax is an ancient disease of animals and man. The causative bacteria can form “endospores”, which are highly resistant to environmental stresses, surviving for decades in soil. In mammals, the spores germinate, spread and release a potentially fatal toxin. In 2001 anthrax spores were sent in letters to scores of US congressmen and media pundits. Five people died. Anthrax has long been considered an option in bioterror and warfare. In 1944 the British contemplated dropping anthrax-impregnated cattle feed into Nazi-occupied Europe to destroy agriculture there.
Michael Barrett is a professor of parasitology at the University of Glasgow<span style="font-size:9.5pt;color:#D71920;text-transform: