How are Diseases Spread

Human history is punctuated by a number of terrifying pandemics. These global outbreaks of disease will, without a doubt, remain a real threat to humanity for the foreseeable future, however our growing knowledge of medicine and how the diseases work makes us better equipped than ever to fight them.

The word ‘disease‘ encompasses the wide range of ailments from which we can suffer. The ones that cause epidemics and pandemics are infectious diseases – those that can spread from one person to another.

Infectious diseases include everything from the common cold to HIV/AIDS. They are caused by biological agents (or pathogens) – frequently bacteria and viruses but also parasites, fungi and prions (such as bovine spongiform encephalopathy, also known as mad cow disease).

Most micro-organisms living either on or inside our bodies are totally harmless. In fact, our bodies contain up to ten times more bacterial cells than they do human cells! This still leaves a good few handfuls of pathogens intent on ensuring their survival at our expense.

Whether it’s the common flu virus or more unusual bugs, these pathogens have evolved a whole arsenal of tricks to hop from one human to another, spreading through coughs and sneezes, bodily fluids and more.

Some even hitchhike on another organism that does all the legwork for them – known as a vector. Malaria is a good example of a vector-dependent disease. Caused by a parasite, malaria is carried by mosquitoes, which spread the illness between humans when they bite them.

Despite these clever tactics, it’s relatively rare for a pathogen to infect enough people to spark a pandemic. One reason is that our bodies possess a highly effective defence against their onslaught: the immune system.

As we’re exposed to diseases, our immune systems develop tailor-made antibodies to latch onto attackers and either neutralise them or earmark them for destruction. Over time, your body builds up a vast catalogue of antibodies. A pathogen’s first attack might cause a fullblown infection, but if a repeat invasion occurs, the immune system responds swiftly to defuse the attack.

As a result (and also thanks to vaccination), a substantial proportion of the population is resistant to common diseases, making it hard for these pathogens to infect enough people at one time to cause a pandemic. Chickenpox, for instance, is highly contagious, but after a person has been attacked by it once, their body ‘remembers’ the intruder and the vast majority are immune to it for life.

Pandemics therefore tend to be triggered by pathogens which we have had very little exposure to and which can catch our immune systems unawares.

Historically, diseases from other countries could do just that. When European settlers arrived in North America, they introduced diseases which Native Americans had never encountered before, such as measles, influenza and smallpox. These pathogens set off waves of deadly epidemics which killed over 90 per cent of the indigenous population.

Nowadays, regular international travel spreads pathogens around, meaning ‘old’ diseases aren’t a threat. The ones to worry about are new diseases (or variants of known diseases), which all come from one source: animals.

The animals most likely to pass on their diseases are our closest relatives, the great apes. The HIV virus, for example, has been traced back to chimpanzees in Africa who were eaten by humans in the first half of the 20th century.

Like many other zoonoses (diseases which cross the species barrier) HIV first infected a few isolated humans, but as the virus evolved it crossed another very important hurdle: it acquired the ability to transmit from one person to another. Once a disease possesses the capability to do this, it really does become a ticking time bomb.

While it’s relatively easy for a pathogen to make the leap from an ape to us, or vice versa, much greater leaps are possible. Very few of us come into contact with apes, but we have a much closer relationship with domestic animals. A precursor to the influenza virus which caused the 1918 Spanish Flu is thought to have existed first in wild birds, then in domestic pigs, before jumping over to people.

For millennia humans were entirely at the mercy of disease, but the late-18th century saw the invention of our best weapon against infection: the vaccine.

Vaccines fool the immune system into thinking it’s being attacked by a pathogen, stimulating it to create an army of antibodies and killer T-cells specific to this disease. If we ever encounter the real thing, our bodies are therefore primed to fight off the infection.

To do this a tiny amount of the pathogen (usually attenuated, which means weakened) is injected into the body – this process is known as inoculation. It might come as a surprise but people were actually experimenting with inoculation in China and India as long ago as 1000 BCE. However, it wasn’t until 1796 that the first successful vaccination was produced.

Throughout the 18th century, smallpox was a leading cause of death around the world. English physician Edward Jenner noticed that milkmaids who had caught an illness called cowpox seemed to be immune to smallpox. By injecting patients with pus taken from cowpox sores, he was able to confirm his hypothesis and the first vaccine was born. Vaccination was adopted across the globe and smallpox was officially eradicated in 1979.

Jenner’s landmark work with smallpox paved the way for a wide variety of vaccines to be created. In the late-19th century, microbiology pioneer Louis Pasteur developed vaccines against anthrax and rabies. A key innovation was that Pasteur’s method consisted of treating pathogens to render them totally harmless.

In the following century, new vaccines were developed at an astonishing rate. American microbiologist Maurice Hilleman alone led the invention of over 30 vaccines (including those against measles, mumps, hepatitis A, hepatitis B and meningitis). Widespread vaccination for common pathogens means that most of the population is now resistant to them, preventing epidemics and forever changing our relationship with diseases which plagued humanity for centuries or even millennia.

But while it is easy to create vaccines against some diseases, others are far more elusive due to the rapid changes they undergo. The most notorious of these shape-shifters is the HIV virus. HIV has one of the highest mutation rates known, reshuffling its genes constantly to change the shape of its surface proteins. By modifying its disguise, it makes itself unrecognizable to antibodies, dodging the immune system’s guardsmen. Developing a vaccine against HIV is therefore a tremendous challenge. HIV also attacks the immune system directly.

Other diseases, such as influenza, are relatively easy to vaccinate against once a new strain has been identified, however the unpredictability of outbreaks means that they are still a real threat. Influenza exists principally in wild birds, but every so often a new strain of the virus will become transmissible between humans, sparking epidemics and even pandemics, such as the HN51 virus. The time and location of these spillovers are virtually impossible to predict, though monitoring wild birds is one way of keeping an eye out for new strains of the virus which have the potential to make the leap.

In a number of ways, modern society leaves us more exposed to pandemics than we were in the past. For one thing, nowadays about 50 per cent of us live in cities where we come into contact with a huge number of people on a daily basis, facilitating the spread of disease.

The ever-growing number of international flights also accelerates transmission, allowing pathogens to hop from one continent to another in a matter of hours. In 2003 SARS (severe acute respiratory syndrome) spread to 29 countries across three continents in just a few months.

Despite this, our understanding of pathogens is continually improving. SARS was a brand-new disease, but the global medical community rapidly got to know its idiosyncrasies and brought it under control. While we don’t have a vaccine for HIV yet, treatments have improved dramatically, and public health measures have helped to reduce or stabilise infection rates in most countries.

Pandemics will inevitably continue to strike in years to come, but we can rest assured that we are better armed than ever in our eternal battle against pathogens.

The Great Plague

Probably the deadliest pandemic in human history, the Great Plague, or Black Death, ravaged Europe from 1348-1350, killing up to half of the population in that short time. At its root was the bubonic plague-causing Yersinia pestis bacterium.

Originating in Asia, the plague first struck in China where it killed approximately 25 million people. It then spread into Europe following the Silk Road. Y pestis bacteria were carried by fleas, who themselves hitchhiked on the rats that thrived in the hulls of merchant ships.

After first landing on the shores of Italy in 1347, the Black Death had swept as far north as England by the following summer and continued onward to Germany and Scandinavia in 1348.

Highly infectious, the disease struck and killed its victims with startling speed. The telltale sign of infection was the appearance of swollen lymph glands – called buboes – typically around the groin, neck and armpits. Sufferers then developed a high fever and began to vomit blood, usually dying within a week of the first symptoms showing. With no effective cure to keep it in check, the plague returned repeatedly for the next 300 years.

Smallpox

Following the discovery of a vaccine this acute and often-fatal contagious disease has now been eradicated. Smallpox develops in those exposed to the Variola virus. Following exposure the virus incubates for a period of usually 12-14 days during which time the individual is not contagious. After incubation, however, the sufferer will experience feverishness, sickness and headaches before developing the dreaded rash. This is when the victim is at their most contagious. The itchy red spots become fluid-filled pustules that scab over. The victim will remain contagious till all the scabs fall off.

Leprosy

Though an ancient illness, leprosy – or Hansen’s disease – remains a highly infectious, chronic ailment today. This isolating disease is caused by a microbacterium in the environment – probably in the form of nasal mucus droplets – which is absorbed into its victim’s bloodstream. Leprosy affects the skin and nerves, causing lesions and patches on the skin as well as loss of sensation and weakness in the hands, feet and face. While historically people believed that the disease caused parts of its sufferers to ‘fall off’ it is in fact the numbness caused by nerve damage that leads leprosy sufferers to injure themselves. For example, they could burn themselves on a flame and not even feel it. Today the disease is curable with the help of antibiotics.

Malaria

Though malaria is not contagious (ie not passed from person to person like a cold) this disease endemic to the tropics is a widespread killer. It’s transmitted by the presence of a certain single-celled parasite in the saliva of female mosquitoes. When the mosquito pierces the skin of her victim with a straw-like proboscis mouthpart to obtain blood, she also releases some saliva which acts as an anticoagulant to keep the blood flowing. In doing so she also passes deadly parasites into the victim’s bloodstream. Flu-like symptoms usually occur between ten days and four weeks after infection, however the parasite can lie dormant in the liver for up to four years before making the victim sick.