The human hand is an incredible tool—and a deadly threat. “There is no act of life so dangerous to others,” fumed physician Robert Eccles in 1909, “as carelessness concerning the condition of our hands.”

He really meant it. In a seven-page rant titled “Dirty Hands,” published in the Dietetic and Hygienic Gazette of New York City, Eccles blamed filthy fingers for the deadliest crimes of the age. Causing more deaths than “bullets, poisons, railway accidents and earthquakes combined,” the human hand was a weapon of mass destruction that extinguished innocent lives by the hour, according to this Brooklyn-based doctor. And Eccles was fighting back. With ample ammunition from research in bacteriology, a field in its heyday by the close of the 19th century, he had scientific proof that uncleanliness could transform hands into petri dishes of pathogens. “Until the HABIT is established of purifying the hands, both timely and properly, no lessening of this human misery seems possible under existing conditions,” Eccles declared.

The main target of the doctor’s ire was a private cook named Mary Mallon, the notorious “Typhoid Mary” of medical lore, who was serving a sentence of forced isolation on North Brother Island in New York City’s East River. Mallon was arrested as a public health threat in 1907 after being identified as the source of seven household outbreaks of typhoid fever since 1900.

Epidemiological evidence suggested that she infected her clients by preparing their meals with unclean hands—a charge that Mallon rejected. She didn’t deny her poor hand hygiene but also failed to see how she could have infected anyone. Typhoid fever has many symptoms, such as a prolonged high fever, headache and malaise, and Mallon had none of them.

The disease is caused by the bacterium Salmonella typhi, which was well-described and identifiable with diagnostic tests by the 1890s. Untreated typhoid fever can be fatal in up to 30 percent of cases, and before the advent of antibiotics, it caused thousands of deaths in the United States each year. Only humans are infected by and transmit the pathogen, usually through food and water contaminated with Salmonella-filled urine or feces. This is likely how Mallon spread the disease given that laboratory analyses of her feces showed pathogens aplenty, which suggested that none of her trips from the bathroom to the kitchen involved soap.

Vilified as “Typhoid Mary” by the press, Mary Mallon was arrested as a public health threat in 1907. -- Fotosearch / Stringer via Getty Images

Mallon refused to believe that she was an asymptomatic carrier of typhoid fever, even after her release in 1910. She continued to cook, but she didn’t adopt the hand-washing habit that Eccles preached. Thus he was probably pleased by the further punishment that she faced for her dirty hands when health authorities tracked her down again. After more people had fallen ill and died from her contaminated cuisine, she was arrested and isolated for a second time in 1915, with a sentence that lasted the rest of her life.

The story of Mallon holds many lessons, and the danger of unclean hands is one of them. But still today, disease risks frequently involve pathogens and routes of transmission that we fail to recognize. I recall when virologist Matt Frieman made this point effectively at a workshop in 2017. The scientists in attendance were invited to present and discuss their research with a group of filmmakers, and Matt’s topic was perfect for a Hollywood movie: deadly viruses that have recently emerged in humans. When Matt finished his presentation, one filmmaker asked him how much we needed to worry about these pathogens at present. You could hear the alarm in her voice. And without missing a beat, Matt replied, “Right now, our most immediate threat is a norovirus outbreak from that jar of cookies by the bathrooms.”

He was right. In our meeting venue, arranged by one of the premier scientific organizations in the United States, there was an inviting jar of chocolate chip cookies on a small table … directly on the path to and from the toilets.

Like Salmonella typhi, norovirus is an intestinal pathogen that’s commonly spread through contaminated food, water and surfaces. It’s one of the world’s leading causes of gastroenteritis (also known as stomach flu) and extremely contagious, partly because a small dose can cause infection. Incredibly, a sick person can shed billions of tiny particles of norovirus in their stool and vomit, and it takes as few as 18 of those particles to infect another person. Norovirus is also highly transmissible because it’s picked up and left all over the place by our grabby hands.

For an example, look to the utterly miserable weekend of an Oregon girls soccer team in 2010. While sharing hotel rooms at an out-of-state tournament, several of the team’s members fell ill with acute gastroenteritis. The first girl to become sick—called the index patient—had used a bathroom where a grocery bag of snacks was being stored. She didn’t actually touch the bag or its contents but instead contaminated their surfaces by vomiting, excreting diarrhea and flushing the toilet—all of which can aerosolize noroviruses, thereby making them airborne. The index patient went home the next morning, but cookies, chips and fresh grapes in the grocery bag were passed around at the team’s lunch that afternoon. Within 48 hours, seven other players and chaperones became sick, too.

Sickness is often a helpful signal of infection. It tells the patient, as well as the rest of us, to steer clear. But like Salmonella typhi, norovirus infections can be contagious without any symptoms at all. People can shed the virus in their feces before they start to feel sick or for weeks after they begin to feel better. Hand washing is therefore one of the simplest and most effective ways to prevent transmission. Placing treats far away from the restrooms is another one.

How our hands work

Our hands wouldn’t work so well as disease vectors if we didn’t use them so much. And we wouldn’t use them so much if there weren’t so much that they can do. So before we delve further into a discussion of how humans give a helping hand to pathogens in their transmission, let’s consider what makes our hands so helpful in the first place.

Put one hand flat on a surface, palm down, and you might be able to make out the contours of 14 short bones called phalanges in your thumb and fingers, in addition to five longer ones in your palm called metacarpals that articulate with your wrist. Eight small wrist bones called carpals are mostly hidden from external view. Some of them are surprisingly charismatic in shape, resembling miniature forms of common objects that range from a boot to a boat. But there’s nothing cute about what they do. These 27 bones give each hand its rigid, knuckled structure, while joined and surrounded with muscles, tendons, ligaments, blood vessels and nerves that connect with other elements of the body and carry out directions from the brain. Together they’re critical components of the anatomical architecture that allows your hand to move.

At each of your fingertips there’s an ever-growing, translucent plate of fibrous protein called keratin, otherwise known as a nail. Although they’re nice for decoration, your nails protect and enhance your sensitivity to touch, too. Flip your hand over, and you can better understand how. The nails provide a hard backing for fibrofatty cushions of flesh at each of your fingertips, five fingertip pads in addition to several palm pads on the underside of each hand. Extremely creased and furrowed, these pulpy little pillows of nerve endings have some of the highest concentrations of receptors in all the skin, making them highly sensitive to sensory stimuli. Try them out with a tap or two—but be careful! Fingertip injuries are potentially debilitating and common, particularly in curious young children who use their hands to explore their environment without realizing the physical dangers involved. Even beyond childhood, through touch sensations and tactile perceptions of temperature, texture and vibration transmitted to the brain, fingers are essential to how most people contact and interact with the external world throughout life.

Human hands have some minor distinctions among primates that make a big difference. The human hand can be distinguished from those of other living apes by a high thumb-to-digit ratio, meaning that we have a relatively long thumb when measured against the fingers on the same hand. One major advantage of these hand proportions is that our thumb can be placed squarely in pad-to-pad contact with, or positioned diametrically opposite to, any or all of our fingers. Thumb opposition isn’t unique to humans, and in fact an opposable thumb facilitates the enhanced grasping abilities of many primates. But what sets our thumb apart is its power. Modern humans have a unique combination and greater number of forearm muscles versus other primates, as well as a notable musculature in the thumb. Altogether, these features allow humans to firmly and precisely grip objects for certain types of manipulation that other animals, even our living primate relatives, can’t achieve.

Imagine pinching a piece of paper between your thumb and index finger, for example. We use this type of forceful, pad-to-pad precision gripping without thinking about it, and literally in a snap. Yet it was a breakthrough in human evolution. Other primates exhibit some kinds of precision grips in the handling and use of objects, but not with the kind of efficient opposition that our hand anatomy allows. In a single hand, humans can easily hold and manipulate objects, even small and delicate ones, while adjusting our fingers to their shape and reorienting them with displacements of our fingertip pads. Our relatively long, powerful thumb and other anatomical attributes, including our flat nails (which nearly all primates possess), make this possible. Just picture trying—and failing—to dog-ear a page in a book with pointy, curved claws.

With a unique combination of traits, the human hand shaped history. No question, stone tools couldn’t have become a keystone of human technology and subsistence without hands that could do the job, along with a nervous system that could regulate and coordinate the necessary signals. Even for those who have never attempted to make a spear tip or arrowhead from a rock (which is most of you), it’s obvious that it would require strong grips, constant rotation and repositioning, and forceful, careful strikes with another hard object. And even for those who have done so, it can be a bloody business.

The Human Disease: How We Create Pandemics, From Our Bodies to Our Beliefs -- A journey through history and around the globe to examine how and why pandemics are an inescapable threat of our own making.

But our manual dexterity isn’t determined by our hand anatomy alone. Our nervous system, which involves the brain, spinal cord and a complex system of nerves, exerts control over our hand movements. Indeed, neurological factors may partly explain why primate species with similar hands can differ quite a bit in their mechanical abilities. For example, the tufted capuchin and common squirrel monkey both have pseudo-opposable thumbs, but only the capuchin displays relatively independent finger movements and precision gripping in picking up small objects and manipulating tools. Functional differences in their neuroanatomy may be the cause.

Of course, the most common object that people touch nowadays is a screen. And the tap-tap-tap movements of our fingers is a unique human ability, as no other primate can move their fingers as rapidly and independently as we do. Here again, we can thank the extraordinary human brain given that normal finger tapping requires the functional integrity of different parts of our central nervous system. Moreover, repetitive rapid finger tapping is a common test of fine motor control of the upper extremities as well as a standard means of assessing the potential effects of neurodegenerative disease and traumatic brain injury.

While a human can turn the page of a book using forceful thumb-finger opposition, other apes can’t form this pad-to-pad “precision grip” due to the relative shortness of the thumb compared to the other fingers, as seen in the left hand of this chimpanzee. Instead, this chimpanzee is gripping the pages of a magazine by holding them between the knuckles of its right hand. -- Mertie . via Flickr under CC By-SA 2.0 Deed

Our use of information technology, like smartphones and computers, is often described as having the world at our fingertips. But this metaphor makes sense when it comes to microbes, too.

Microbes and our hands

The vast majority of microbes on and in the human body are persistent but harmless colonists. Those on the hand are no exception.

Many of the microbes at our fingertips provide important benefits for human health. For instance, one of the key functions of the skin microbiota, which are mostly bacteria, is acid resistance. By regulating the acidity of the skin, these microbes help to maintain a powerful permeability barrier that prevents water and electrolyte loss from the body—a requirement for life in terrestrial animals like us.

Our skin barrier also prevents infectious diseases and allergies by blocking external substances such as pathogens, allergens and chemicals from invading the body.

At least that’s how the barrier is supposed to work. But even though many of the microbes that come in contact with or reside on the skin are normally unable to establish an infection, any break in the skin from a cut, scrape, burn or bite can be the entry point of an invading pathogen, such as Ebola virus from the infected blood of a mammalian host or Zika virus from the infected saliva of a mosquito vector.

But these aren’t the most frequent ways that our hands participate in the spread of infectious diseases. Rather, our hands are critical in the indirect transmission of pathogens between people via contaminated objects and surfaces, as Mary Mallon did throughout her career. Called fomites, these risky objects are everywhere: phones, faucets, doorknobs, elevator buttons, dishtowels, utensils, food, you name it. We touch these things and the microbes on them literally all the time.

Parents won’t be surprised that children can touch objects and surfaces more than 600 times per hour during outdoor play. At the same time, these little explorers might touch their mouths or someone else’s about 20 times an hour. Yet adults do this quite a bit, too. Regardless of age or sex, we might touch our faces up to 800 times a day. Often the touch comes from an automatic and unconscious movement, and so if you think you’re an exception, it could be that you simply don’t remember. For instance, when prompted to recall nonverbal behaviors during interpersonal interactions, the subjects of one study showed the lowest accuracy in estimating how many self-touches they made.

Hand contact with the mouth, nose and eyes—sometimes called the facial T-zone by infectious disease researchers—is the riskiest kind of face touching. That’s because the mucous membranes that line these structures can serve as staging grounds for microbial pathogenesis, the process by which microbes cause disease. People have been observed touching their T-zone around eight times an hour in public places, and the number nearly doubles for kids. In medical offices, some health care workers make T-zone touches with the same frequency as people do in public, although clinicians do so slightly less often. But believe it or not, medical students can be even worse. In one study, they were observed touching their face 23 times per hour while listening to a lecture—after completing coursework in infection control and transmission precautions, no less. And almost half of those touches involved contact with a mucous membrane.

Hand contacts with fomites and mucous membranes are a potentially dangerous combination. People who are infected with pathogens can expel them from their bodies in saliva, mucus, blood, urine and feces as well as in respiratory secretions in the form of droplets and aerosols. These pathogens can be deposited on or transferred to fomites in a variety of ways, from an explosive sneeze or casual touch. Then the pathogens can survive and remain infectious on fomites for varying lengths of time, from a few hours in some cases to several months in others depending on variables related to the pathogen, the fomite and their environmental conditions. Many people were made aware of these possibilities during the Covid-19 pandemic, when the earliest recommendations from health officials included washing your hands, cleaning surfaces and not touching your face.

Some pathogens are more likely than others to spread via fomite and hand-to-hand contact, even if SARS-CoV-2 doesn’t appear to be one of them.

This is the case for some gastrointestinal pathogens like Salmonella typhi, norovirus and poliovirus, which usually follow a route of fecal-oral transmission. Others such as Vibrio cholerae (bacteria that cause cholera) and Escherichia coli (bacteria that can cause a variety of infections depending on the strain) are more likely to spread through fecal contamination of food and water.

But fomite-mediated transmission is also a concern for some respiratory pathogens like rhinovirus, which is the predominant cause of the common cold. One study found that around 14 percent of the rhinovirus on an individual’s fingers was transferred to another individual via a doorknob or faucet, and half as much via hand-to-hand contact. Furthermore, another study found that after an overnight stay in a hotel, adults with natural rhinovirus colds contaminated about 35 percent of the 150 environmental sites tested, such as pens, light switches, remote controls and telephones.

In one-third of the trials, the study’s subjects indirectly transferred the virus to other people’s fingertips up to 18 hours after contaminating these surfaces. If this isn’t an argument for hand hygiene, then I don’t know what is.

And this argument long preceded Mallon.

In 1847, when Hungarian physician Ignaz Semmelweis devised the interventions that would earn him the title of “the father of hand hygiene,” the discipline of medicine was on the verge of a revolution. Surgeons had just started using general anesthesia when operating on patients, who were able to experience painless operations as never before. Anesthesia was also first used for childbirth in 1845, at a time when maternal death was far too common; in general, for every thousand babies born during the 19th century, as many as ten mothers died. One of the major causes of maternal mortality was childbirth-related septicemia, known as puerperal fever or childbed fever—later found to be caused by Streptococcus pyogenes bacteria. Between 1841 and 1847, puerperal fever was responsible for up to 16 percent of maternal deaths at the hospital in Vienna, where Semmelweis worked. Mothers died far more frequently, however, in one of the hospital’s obstetric wards than in the other one. And Semmelweis seized the opportunity to understand why and how.

He examined the mortality statistics at the hospital over decades, finding that the mortality rates of the two wards diverged after 1841. At that time, one of the wards became staffed only with midwives. In the other one, deliveries were performed by medical students and doctors, who also conducted autopsies in a nearby room. After one of the hospital’s pathologists died following a scalpel slip during an autopsy, from which he succumbed to a condition similar to puerperal fever, Semmelweis made the cadaver connection.

Concluding that the medical students and obstetricians were causing puerperal fever in their pregnant patients by infecting them with cadaverous particles on their hands, Semmelweis instituted some harsh protocols. Everyone had to scrub their hands with a chlorinated lime solution after leaving the autopsy room and before contact with a patient. Why chlorinated lime? Because Semmelweis didn’t think that soap and water were strong enough to remove the culprits of contagion from post-autopsy hands, and chlorinated lime solution was the strongest product used by the housekeeping staff at the hospital.

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