The common cold can be spread through just one sneeze, and that single sneeze can let fly as many as 100,000 germs traveling at about 100 miles (161 kilometers) per hour. And although in the 1960s and '70s we believed we were safe from germ transmission as long as we had buffer of 3 feet (1 meter) from said sneeze, recent research suggests the droplet-spray radius is actually five to 200 times larger than that [sources: Hatfield, Bourouiba].
Millions of people suffer through an average of two or three colds every year, which, when you think about how each round lasts anywhere from a week to 10 days, is nothing to sneeze at — and that's with an understanding of how it's spread. We used to think that the common cold was caused by being in a cold environment. We got that one wrong, didn't we?
Infectious diseases are bacterial, viral, parasitic and fungal infections that spread among us through direct, person-to-person or indirect contact, not through witchcraft, punishment for sin, a problem of the body's "humours" or any other mystical force. Direct contact transmission refers to diseases, such as human papillomavirus (HPV) and the Ebola virus, that need physical contact to spread from person to person (or, for instance, direct contact with an infected animal, such as a bite from a Borrelia burgdorferi-infected tick leading to Lyme disease). Indirect contact, on the other hand, doesn't require any skin-to-skin contact; MRSA, for instance, is able to linger on a keyboard for weeks after an infected person typed on it. And measles, spread through airborne transmission, similarly lingers for hours outside the body, except instead of remaining on objects, these germs remain suspended in the air around us.
But let's set all that aside and look at the more supernatural and moral meanings we once used — and sometimes still do — to explain certain diseases.
We didn't always know that Chlamydia trachomatis, a bacterial pathogen, causes chlamydia, nor did we know it's a sexually transmitted infection.
Chlamydiae are microorganisms, and they're unique because they have both viral and bacterial properties. Like a virus, they need a host cell in which to grow — although initially because the microorganism relies upon a host, it was first believed to be a protozoan parasite. Neither a parasite nor a virus, chlamydiae, just like gram-negative bacteria, have DNA and RNA, replicate through binary fission and have cell walls with an outer membrane.
There are multiple strains of C. trachomatis, and not all of those strains cause genital tract infections. Through hand-to-eye contact, for instance, some strains of C. trachomatis may cause respiratory and eye infections, such as severe chlamydial conjunctivitis, neonatal conjunctivitis or a chronic infection called trachoma. As long ago as 15 B.C.E., both ancient Egyptian and Chinese writings describe a chlamydialike infection among diseases of the eye, not the genitals, and it wasn't until 1976 that chlamydia was recognized as an STD [source: CDC, Black].
Today, chlamydia is one of the most common bacterial infections and the most frequently reported STD in the U.S. Nearly 1.5 million cases of genitourinary chlamydia infections were reported in 2013. But because up to half of infected males are asymptomatic, and as many as 70 percent of women don't experience symptoms, health professionals suggest the actual number of people infected is closer to 3 million [sources: HRF, Black]. When symptoms are present they commonly include abnormal discharge and painful urination. Chlamydia is treatable with antibiotics; left untreated, it may cause long-term complications such as infertility and chronic pelvic pain.
The ancient Egyptians described malaria in the Ebers Papyrus 4,000 years ago, although evidence in 30-million-year-old fossils suggests the disease has been with us for much longer. In 400 B.C.E., Greek physician Hippocrates described malaria's symptoms and characteristics in his treatise "On Airs, Waters, and Places." During the first half of the first century in the Roman Empire, Celsus left behind clinical descriptions of it. And during the third century, Quintus Serenus Sammonicus, physician to the Roman emperor Caracalla, prescribed wearing an amulet inscribed with the word "abracadabra" for nine days as treatment.
For thousands of years we've known malaria to be a contagious and fatal condition, but it took us a really long time to figure out how it spreads among people. Hippocrates, for example, blamed it on unhealthy air and poisonous gasses; the disease was nicknamed "marsh fever." It's also been associated with supernatural causes.
We didn't know the true cause of malaria until 1880, when French scientist Alphonse Laveran discovered that it's a parasitic infection. Specifically, it's the Plasmodium parasite that's responsible for the disease, and there are four species that infect humans: P. falciparum and P. vivax (the two most common) and P. malariae and P. ovale. It would be nearly 20 more years before scientists discovered this blood parasite is transmitted through the bite of infected female Anopheles mosquitoes. In 1902 Ronald Ross won the Nobel Prize for Physiology or Medicine for his discovery that the Plasmodium parasite lives and breeds in the gastrointestinal system of mosquitoes.
During the time of the Black Death in the 14th century, plague was believed to be caused and spread by miasmas — unhealthy, disease-carrying smells and vapors from either the corpses of those who died by the disease or from the breath of those infected. Throughout the centuries, plague has been considered God's punishment for sins, and it's also been blamed on astrological and natural phenomena, such as comets and earthquakes. And to treat it, people have tried aromatic vapors from flowers and herbs, such as rose and thyme, as well as amulets and magical remedies. They've asked patron saints for help and tried to purge their sins through self-flagellation and prayers.
The real causative agent of the plague wasn't discovered until 1894, during the third worldwide pandemic, when bacteriologist Alexandre Yersin identified the bacterium Yersinia Pestis. Today we know that Y. pestis can infect different parts of the body, resulting in three types of plague: bubonic (infecting the lymphatic system), pneumonic (infecting the lungs) and septicemic (infecting the blood).
Yersin also discovered that rat fleas — specifically the Oriental or Indian rat flea Xenopsylla cheopis, and the Northern or European rat flea Nosopsyllus fasciatus — were the primary carriers and spreaders (called primary vectors) of the bacterium during the pandemic. The human flea, Pulex irritans, along with the dog and cat fleas, Ctenocephalides canis and Ctenocephalides felis, were secondary vectors. It's theorized that gerbils may have been responsible for introducing plague-carrying fleas on trade routes from China.
Bubonic and septicemic types of plague spread through flea bites from infected fleas or, though more rarely, through bites from an infected animal, such as a rat or gerbil. Pneumonic plague, however, is directly spread through infected droplets from sneezing and coughing.
While infected fleas are known vectors of Y. pestis, it's Y. pestis itself that gave rise to the possibility of plague. The bacterium was originally a gastrointestinal pathogen, but researchers discovered in 2015 that during its evolution, the bacterium underwent two genetic changes that transformed it from a mild stomach bug to a pandemic-causing plague agent. In its first evolution, it acquired through a single gene the ability to produce a protein that gave it the ability to infect the lungs. Through a single genetic mutation of that acquired gene, Y. pestis next evolved the ability to infect the blood and the lymphatic system.
The World Health Organization recommends three basic ways to avoid the spread of Ebola: Don't touch sick people; avoid bodily fluids and dead bodies; and be vigilant about hand washing. If you're wondering where the hazmat suit falls into that equation, it doesn't. The Ebola virus is not airborne, it's not waterborne and it doesn't spread through casual contact with another person. That means that most of us not living in developing countries or caring for infected patients on the front line of an Ebola epidemic are at little to no risk of becoming infected as we go about our day-to-day lives.
Ebola, caused by a virus of the family Filoviridae, genus Ebolavirus, was first identified in 1976 in what is now the Democratic Republic of Congo [source: McLysaght]. Four of the five known Ebolavirus strains infect humans: Bundibugyo virus, Ebola virus, Sudan virus and Taï Forest virus. The fifth strain, Reston ebolavirus, is known to cause infections in monkeys, gorillas and chimpanzees.
There is also no evidence to suggest the Ebola viruses are genetically modified organisms, a bioweapon, or a government plot devised to wipe out a certain race or group of people. Some have voiced suspicions that health care workers at local hospitals, including local physicians and teams from Médecins Sans Frontières (aka Doctors Without Borders), are responsible for the introduction and spread of the virus.
Additionally, there is no evidence Ebola can be prevented or cured by consuming coffee, condensed milk or raw onions. Kissing the corpse of an infected person will not grant you immunity; in fact, that's a good way to become infected.
Because antibiotics work against bacteria and Ebola is a viral infection, antibiotics aren't useful against the disease. As of mid-2015, many Ebola vaccines are being researched and tested, but there is no official vaccine or cure for Ebola.
The human immunodeficiency virus (HIV) is a lentivirus that attacks the immune system. Because it has a long incubation period, it may be a decade before an infected person shows any symptoms. HIV is the virus that causes acquired immune deficiency syndrome (AIDS), and, because of its destruction of the immune system, it frequently also leads to other opportunistic infections. Although there are recorded instances of single cases of infection reported in the mid-20th century, the first case of today's HIV/AIDS epidemic was reported in 1981. By 2014 more than 35 million people were living with HIV worldwide [source: amfAR].
While we still can't be positive, scientists have a leading theory of HIV's origins. According to the "hunter" theory, Simian Immunodeficiency Virus (SIV), the type that infects primates, jumped from primates to humans through the butchering and eating bushmeat; this strain would become known as HIV-1. Other theories remain controversial or have been debunked, including the idea that the virus was transferred through the oral polio vaccine or through sharing contaminated needles during inoculations. It's also been suggested that HIV is a genetically modified organism and that it's a government conspiracy against black and gay populations.
Not only did we not know the origin of the virus early on, we didn't even know how it spread. Some thought HIV spread just by touching an infected person, or by touching an object after an HIV-positive person had. That's incorrect. You're not at risk through handshakes, hugs, kisses, sharing a toilet, using the same exercise equipment or touching a doorknob. Some thought HIV spread through saliva, sweat and tears — it doesn't. In the early '80s, a Canadian flight attendant named Gaetan Dugas was fingered as "Patient Zero" of the HIV epidemic because of his extensive travels and HIV-positive status. But he wasn't; the HIV entered the U.S. long before his career in travel began. Nor is there any evidence the virus is passed through mosquito bites. And since 1985 the Public Health Service has been screening donated blood for HIV. And no, HIV is not airborne.
Despite the myths about HIV transmission, the truth is that the virus spreads only through contact with certain HIV-infected bodily fluids, including breast milk, blood, semen and vaginal fluid, and often through unsafe sex or through sharing contaminated needles.
Vector-borne diseases are those spread by biting insects. HowStuffWorks looks at the alarming rise in infections.
Author's Note: 5 Diseases That Don't Spread the Way We Used to Think
When I hear "abracadabra" — which, to be honest, really isn't often, now that I think about it — I guess I imagine the stereotypical magician pulling a bunny out of a hat scenario, eh, or maybe The Steve Miller Band. So it was interesting to learn the word actually existed long ago when it was inscribed on amulets and used as an incantation to ward off illness and other misfortunes.
More Great Links
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