Infectious diseases are the terrorists of the natural world.
Most of them, though, quite frankly, aren't very good at inspiring terror. The common cold? An annoyance, but not a terror. The chicken pox? Itchy, but not terrifying. Even a nasty flu is rarely more than acute unpleasantness.
But then there are the few that really stand out -- the diseases that make us all a little more appreciative of our health and a little more respectful of the microbial world.
No. 01 - HIV/AIDS
HIV/AIDs is scary.
It's the scourge of our time. And what it does is pretty scary, too. HIV itself doesn't kill you; it lets other infections do the dirty work.
The virus attaches to immune system cells, particularly the helper T cells, which regulate much of the body's immune response. From there, the retrovirus uses your cells to replicate, killing many of those infected host cells in the process.
HIV takes time to do its work: years can pass before the virus wreaks enough havoc to cause AIDS, the syndrome that inspires fear worldwide. Though antiretroviral drugs can prolong life and delay the onset of AIDS, they're often prohibitively expensive, meaning that many patients are left to succumb to the opportunistic infections that will ravage their weakened bodies.
Imagine going up against pneumonia, tuberculosis, MRSA, herpes viruses, skin infections and even common respiratory ailments with no ability to fight back.
Now that's scary, and it'll make you appreciate how amazing your (healthy) immune system really is.
No. 02 - Ebola, Hanta & Hemorrhagic Fevers
Drowning isn't fun; neither is bleeding. Hemorrhagic fevers find a way to mix the two: as the entire body oozes and bleeds, the lungs can bleed into themselves, causing some victims to literally drown in their own blood.
Hemorrhagic fevers are the result of some very scary viruses like Ebola and Hanta, most of which are spread via animal feces and transmitted either through the air or through direct contact.
Once in the body, the viruses attack the cells that line blood vessels, causing internal organs throughout the body, from the intestines to the kidneys to the brain, to ooze blood.
There's no band-aid big enough to stop that bleeding.
No. 03 - Rabies
Foaming at the mouth, difficulty swallowing, a maniacal fear of water, anger and hostility, delusions and hallucinations, general all-round insanity.
You may recognize these symptoms from that pack of rabid raccoons in your backyard, but a couple of hundred years ago, a lot more humans found themselves acting like those unfortunate animals.
Back before Louis Pasteur's groundbreaking vaccine hit the scene in 1885, rabies was a widely feared disease (and it still is in some parts of the world). Spread through saliva (usually through dog bites), the rabies virus attacks the nervous system; once it's gotten to your brain, it's pretty much over.
Today, mandatory animal vaccination programs have pretty much wiped out the disease in humans in the developed world, but the disease still kills millions of animals and up to 50,000 people worldwide each year. Consider that a reminder to vaccinate Fluffy.
No. 04 - Antibiotic-Resistant Bacteria
A few decades ago, we got all cocky and thought we had bacteria beat.
"Antibiotics!" we said. "Take that, bacteria!" And then, one day, they laughed at us.
Little did we know that bacteria had their own set of tricks up their sleeves (or would have, if they had sleeves). As common bacteria evolved resistance to antibiotics, many of them became real threats, colonizing wounds and spreading in hospitals full of immune-compromised patients. Hospital-borne bacteria like MRSA (methicillin-resistant Staphylococcus aureus),Acinetobacter baumannii and Pseudomonas aeruginosa can spread through the body, leading to pneumonia and sepsis (a whole body, systemic infection).
MRSA in particular has gotten a lot of notoriety recently for its ability to chomp through flesh (doctors call that symptom "necrotozing faciitis"), while Acinetobacter baumannii gained the nickname "Iraqibacter" for its prevalence among wounded Iraq War veterans. Pseudomonas aeruginosa, for its part, made the news recently when it infected a Brazilian beauty queen and caused her to lose her hands and feet, and then her life. Scary stuff.
No. 05 - Naegleria (brain-eating amoeba)
Don't drink the water. And while you're at it, you might not want to take any chances by putting your head in it, either.
Naegleria fowleri, a not-so-friendly little amoeba, makes its home in warm fresh water in the American Southwest. That's not such a problem, but it occasionally also likes to make its home in people's brains, which is a bit of a problem.
The amoeba usually creeps through your nose while you swim in lakes or hot springs. Once it finds its way up into your brain, it's pretty much over. The seizures start, followed by a coma.
The parasitic amoeba chews through your brain matter, and since you kind of need your brain to live, that means it's curtains for both you and your parasitic, opportunistic amoeba friends.
No. 06 - Mad-Cow Disease
Mad-cow disease, also known as bovine spongiform encephalopathy (the spongiform is for sponge, as in "this disease will chew holes in your brain until it looks like a sponge"), first came to the public's attention in the mid-1990s, when the illness, caused by deformed protein fragments called prions, made the jump from cows to humans.
Known in humans as variant Creutzfeldt-Jakob disease, the illness is spread through contaminated meat and causes a host of awful degenerative neurological symptoms, including dementia, loss of nervous system and muscle control, and eventually, death.
And as if the idea of your brain slowly turning to Swiss cheese isn't gross enough, its path to prominence is also disgusting.
The dangerous prion spread to outbreak proportions due to the then-common cannibalistic practice of feeding diseased cattle remains (including bone and brain, complete with BSE prions) to other cattle, which were then consumed by humans. Now, that's just wrong.
No. 07 - Leprosy
Lepers (and leprosy) have been around since biblical times; so has the stigma against them.
For centuries, the disease was believed to be a curse. Stories abounded about the terrifying symptoms: skin lesions turn to dying flesh and into fallen limbs.
Yes, that's right. Your leg might just fall off. Eek!
Suddenly, those leper colonies make a lot more sense. In reality, leprosy doesn't actually cause limbs to drop left and right.
The disease, also known as Hansen's disease, is caused by Mycobacterium leprae, a bacteria that infects the peripheral nerves. Without functioning nerves to feel pain and temperature, patients can often inadvertently injure themselves and opportunistic infections can take hold, sometimes leading to the loss of a finger or toe (hence, the fallen limb rumors).
It's not a highly infectious disease, but can spread in areas with poor hygiene and close living conditions. Things changed a lot for lepers in the 1950s with the rise of antibiotics such as dapsone.
Today, we treat it with a multi-drug regimen and, though colonies still exist, far fewer people lose life or limb to the disease.
No. 08 - Botulism
For something that women cheerfully inject into their foreheads every day, botulism neurotoxin (BT) sure is scary.
BT is the handiwork of the common soil bacteria Clostridium botulinum. The bacteria can be spread through food contaminated with the bacteria or its spores, or through an open wound.
Once in the body, the bacteria start producing the toxin and things get messy. Within a day or two, neurological symptoms appear, including slurred speech, blurred vision and trouble breathing. Muscles get weaker, reflexes stop working, limbs get paralyzed.
Eventually, the diaphragm and other breathing muscles stop working, causing death. Antitoxin and antibiotics can halt the disease's progress, if administered in time, but it can take months to fully recover from the paralysis.
No. 09 - Elephantiasis
It turns out that the famous "Elephant Man" did not, in fact, actually have elephantiasis, the disease characterized by acute swelling of the limbs and caused most often by a parasitic worm transmitted by mosquitoes.
What the elephant man did have was some of the classic elephantiasis symptoms, and those aren't pretty. In fact, they're pretty ugly.
The parasitic worms that cause elephantiasis hang out in the lymph system, which controls the immune response and fluid retention — hence the swelling. That swelling most often occurs in the legs, but it can also affect the arms, breasts or even the genitals, causing them to swell and deform to enormous sizes.
Don't get the wrong idea, guys: in men, the scrotum can swell so much that the penis can retract inside the swollen, thickened skin. Now, that just sounds unpleasant.
No. 10 - Polio
Any disease that can paralyze you is scary.
Though it's been around since ancient times, polio, which targets the nervous system, only rose to epidemic proportions in the 20th century, perhaps because increased population density facilitated its transmission.
At its worst, polio was serious business: some patients were left with life-long limb paralysis or deformities, while an even unluckier few found their respiratory muscles paralyzed and were left dependent upon coffin-like iron lungs for their survival. Many eventually died. Amazingly, up to 95 percent of people infected with polio showed no symptoms, and most others just faced your standard flu-like fare … but those numbers can be misleading.
At the height of the polio epidemic in the 1950s, there were over 13,000 cases involving paralysis and 1,000 deaths each year from the disease, many of them children.
Thanks to a large-scale vaccination campaign, polio is no longer a problem in most of the world, but remains endemic in parts of India, Pakistan and Nigeria.