Infection

• 1. Introduction

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  Infection killed approximately 9.2 million people in 2013, which was 17 percent of all deaths that year. The number describes a single biological event repeated billions of times. Pathogens invade tissue, multiply, and the host's body reacts to both the invader and the toxins it produces. That reaction is what we feel as illness. Yet a strange truth runs underneath all of it. An infection is not the same thing as an infectious disease. Some infections cause no illness at all. Less than 5 percent of people infected with polio ever develop the disease. Meanwhile the prion behind mad cow disease and Creutzfeldt-Jakob disease kills every animal and person it infects, without exception. How can the same word cover both the harmless passenger and the certain killer? What decides whether a microbe living quietly on your skin stays a neighbor or becomes a threat? And how did humanity move from blaming bad air to reading a pathogen's genetic code? The answers reach from the colon of a healthy person to the surface of bacteria grown aboard a Space Shuttle.

• 2. The Crowded Body

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  Every multicellular organism is colonized by outside organisms, and most of them mean no harm. Various species of staphylococcus live on human skin in a commensal relationship, taking without harming. Anaerobic bacteria colonize the mammalian colon in a mutualistic arrangement, where both sides benefit. Neither case counts as an infection. Corynebacteria and Viridans streptococci go further still. They actively block pathogenic bacteria from sticking and settling, speeding wound healing in a symbiotic deal with the host. Colonization begins when an organism enters the body, grows, and multiplies, usually slipping in through the mucosa of the mouth, nose, eyes, genitalia, anus, or an open wound. Gas chromatography-mass spectrometry, 16S ribosomal RNA analysis, and other recent technologies have revealed something humbling. Microbial colonization is common even in places people imagine as nearly sterile. The line between colonization and infection is often only a matter of circumstance. Several staphylococcal species stay harmless on the skin, yet in a normally sterile space like a joint capsule or the peritoneum they multiply without resistance and cause harm. The outcome depends on the route of entry, the organism's intrinsic virulence, the size of the initial inoculant, and the immune status of the host. People with compromised or weakened immune systems face higher susceptibility to chronic and opportunistic infections, the kind that wait for a door to open.

• 3. When The Body Loses

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  Clostridium tetani releases a toxin that paralyzes muscles, turning a microbe's chemistry into a weapon against the host. Disease arises when the host's protective immune mechanisms are compromised and the organism inflicts damage, often by releasing toxins or destructive enzymes. Staphylococcus releases toxins that produce shock and sepsis. The herpes virus shows how an infection can refuse to leave, hiding in nerves and reactivating when specific circumstances arise. Persistent infections continue because the body cannot clear the organism after the initial attack, lingering as latent infection with occasional relapses of active disease. Some viruses, once acquired, never leave the body at all. These chronic infections by parasites carry high morbidity and mortality in many underdeveloped countries and cause millions of deaths globally each year. The immune response itself can turn dangerous. The body's reaction to a microorganism often brings high fever and inflammation, and this response has the potential to be more devastating than the direct damage the microbe causes. Pain becomes a telling clue. A review in the Journal of the American Medical Association's Rational Clinical Examination Series found that an increase in pain made infection much more likely, with a likelihood ratio range of 11 to 20, while the absence of pain did not rule infection out.

• 4. Reading The Invader

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  More than half of cases of encephalitis, a severe illness affecting the brain, remain undiagnosed even after extensive testing with microbiological culture and state-of-the-art laboratory methods. Diagnosis often begins far more simply, with medical history and physical examination, and many minor infections like warts and respiratory infections are treated without ever naming the agent. When deeper identification is needed, culture lets organisms reveal themselves. A single bacterium grown on nutrient agar becomes a visible mound called a colony, whose size, color, shape, and form mark the species and strain. Viruses, which cannot grow alone, are grown in cultured cells, where a region of dead cells from viral growth is called a plaque. Some organisms resist easy culture entirely. Treponema pallidum, the spirochete behind syphilis, cannot be cultured in vitro, though it can be grown in rabbit testes. Microscopy carries much of the diagnostic weight, often paired with staining. The Gram stain sorts the bacterial groups Bacillota and Actinomycetota, while the acid-fast stain identifies Mycobacterium and Nocardia. There is even xenodiagnosis, the use of a vector to support a pathogen's growth. For Chagas disease, an uninfected triatomine bug takes a blood meal from a suspected patient, and is later inspected for Trypanosoma cruzi in its gut. The polymerase chain reaction promises to become a near-ubiquitous gold standard, detecting an agent by amplifying its nucleic acids. Metagenomic sequencing aims further, replacing specific primers with untargeted whole genome amplification to identify almost any pathogen in a single test, and reading antimicrobial resistance genes along the way.

• 5. How Disease Travels

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  A cough or a sneeze sends microorganisms suspended in warm, moist droplets toward another person's nose, mouth, or eyes, the route behind airborne disease. The chain of infection links an infectious agent, a reservoir, a susceptible host, and the exit and transmission to new hosts, and every link must fall into place in order for infection to develop. Fecal-oral transmission spreads pathogens like Vibrio cholerae, Giardia, rotaviruses, and Entamoeba histolytica when food or water is contaminated, often causing gastroenteritis. Vertical transmission passes directly from a mother to an embryo, fetus, or baby during pregnancy or childbirth. Vector-borne transmission relies on an organism that carries pathogens between hosts without falling ill itself. Virulence and transmissibility do not track each other in any simple way. Human strains of Ebola virus incapacitate victims quickly and kill them soon after, leaving little chance to travel, and the early stage spreads poorly because victims experience only internal hemorrhaging. HIV does the opposite, killing slowly by attacking the immune system, so many carriers pass it on before they even know they are infected, and its low virulence lets them travel long distances. Epidemics often move through small-world networks, with dense interactions inside hubs of infected people and separate hubs of susceptible people. Cutting the rare jumps between hubs can drastically lower infection rates. Needle exchange programs in areas with high HIV-positive drug use put this into practice, as did the ring culling of livestock used against foot-and-mouth virus in 2001.

• 6. From Bad Air To Penicillin

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  The Greek historian Thucydides, who lived around 460 to 400 BCE, was the first to write that diseases could spread from an infected person to others, recording it in his account of the plague of Athens. Galen, the Greco-Roman physician, speculated around 175 AD in On the Different Types of Fever that plagues spread by certain seeds of plague present in the air. The Sushruta Samhita, dated to about the sixth century BC, held that leprosy, fever, consumption, and other infectious diseases spread between people through contact, shared meals, and shared clothing. The Persian physician Ibn Sina, known in Europe as Avicenna, proposed a basic contagion theory in The Canon of Medicine in 1025, a text that remained Europe's most authoritative medical book until the 16th century. When the Black Death reached Al-Andalus in the 14th century, the Arab physicians Ibn Khatima and Ibn al-Khatib hypothesized that infectious diseases were caused by minute bodies carried through garments, vessels, and earrings. Anton van Leeuwenhoek, who lived from 1632 to 1723, became the first to observe microorganisms, making bacteria visible at last. In the mid-19th century John Snow and William Budd demonstrated the contagiousness of typhoid and cholera through contaminated water, cutting cholera epidemics in their towns. Louis Pasteur proved certain diseases are caused by infectious agents and developed a vaccine for rabies. Alexander Fleming discovered penicillin, the world's first antibiotic, which Florey and Chain then developed, while Gerhard Domagk produced sulphonamides, the first broad spectrum synthetic antibacterials.

• 7. Plagues In Human History

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  The Plague of Justinian, from 541 to 542, killed between 50 and 60 percent of Europe's population. The Black Death of 1347 to 1352 killed 25 million people in Europe over five years and reduced the old world population from an estimated 450 million to between 350 and 375 million during the 14th century. European explorers carried smallpox, measles, and typhus to Central and South America in the 15th and 16th centuries, and between 1518 and 1568 disease pandemics are said to have cut Mexico's population from 20 million to 3 million. Smallpox killed an estimated 60 million Europeans during the 18th century, roughly 400,000 each year, with up to 30 percent of the infected dying, including 80 percent of children under 5. In the 19th century tuberculosis killed an estimated one-quarter of Europe's adult population, and by 1918 one in six deaths in France were still caused by TB. The Influenza Pandemic of 1918, the Spanish flu, killed 25 to 50 million people, about 2 percent of a world population of 1.7 billion. By contrast, influenza today kills about 250,000 to 500,000 worldwide each year. In 2021, COVID-19 directly caused 8.7 million deaths, making it one of the leading causes of mortality worldwide. An estimated 1,680 million people died of infectious diseases in the 20th century, a toll measured in death certificates the World Health Organization sorts by International Classification of Disease code.

• 8. The Edges Of Infection

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  A 2006 Space Shuttle experiment found that Salmonella typhimurium, a bacterium that can cause food poisoning, became more virulent when cultivated in space. On the 29th of April 2013, scientists at Rensselaer Polytechnic Institute, funded by NASA, reported that microbes aboard the International Space Station seemed to adapt to the space environment in ways not observed on Earth, ways that can lead to increases in growth and virulence. By 2017, bacteria were found to be more resistant to antibiotics and to thrive in near-weightlessness, and microorganisms have even survived the vacuum of outer space. Infection also reaches deep into the past. A skull attributed to the early carnivorous dinosaur Herrerasaurus ischigualastensis shows pit-like wounds ringed by swollen, porous bone, marks of a short-lived, non-lethal infection that scientists think came from a fight with another Herrerasaurus. Acrocanthosaurus, Allosaurus, and Tyrannosaurus carry similar documented evidence. Human genetics shapes the present-day story too. Up to 40 percent of SARS-CoV-2 infections may be asymptomatic, and autoantibodies against type I interferons were found in up to 13.7 percent of patients with life-threatening COVID-19. Mutations in the ERAP2 gene tell an older tale. People who inherited two complete copies of the gene were twice as likely to have survived the plague, caused by Yersinia pestis, as those who inherited two truncated copies, a difference written into the genome by death.