10 Deadly Agents the CDC Works With

As kids, we used to live in fear of dog bites because, beyond the painful snap and crunch, we knew something far more horrifying waited: rabies shots, 20-some-odd punctures with giant needles, delivered straight to the abdomen. Utterly cringeworthy, true, but little did we know that, next to the alternative, they were a walk in the dog park.

Following an incubation period of typically three to 12 weeks, the first sign of rabies is often a numb or tingly sensation around the bite, growing worse over the days to come. This prodromal or early symptom stage can last 0-10 days before giving way to two to seven days of acute symptoms, beginning with fever, headache, weakness and discomfort. These then progress to such effects as anxiety, confusion, insomnia, hallucinations, partial paralysis, hypersalivation and difficulty swallowing. Once acute symptoms appear, coma and death follow rapidly, usually within a week.

Rabies kills more than 55,000 people annually, mainly in Africa and Asia, but is 100 percent preventable with proper early treatment. As for post-exposure vaccinations, doctors no longer deliver them to the stomach. Typically, victims receive several rabies vaccine shots over four days, spaced out, along with a few doses of human rabies immune globulin, one at the bite site and the rest pumped in intramuscularly at another site distant from it [source: CDC].

Three words you never want to hear spoken together: brain-eating amoeba. But that's the ghoulishly glib name given to Naegleria fowleri, a mind-munching microbe that causes brain-destroying meningoencephalitis, a disease similar to bacterial meningitis. Its mortality rate -- more than 97 percent of its victims die, usually within days -- is matched only by its rarity: From 2004-2013, only 34 such infections were reported in the U.S. [source: CDC].

Although the amoeba lives in warm fresh water and soil all over the U.S., the infection only occurs when tainted water makes its way deep into the nose, as during irrigation with neti pots, religious head-dunking, or swimming or diving in lakes, rivers or underchlorinated swimming pools. Early symptoms, which kick in one to seven days after infection, resemble those of bacterial meningitis and can involve headache, fever, nausea or vomiting. Later, symptoms progress to stiff neck, confusion, hallucinations, balance difficulties, a tendency to pay less attention to people and environs, and seizures. Once symptoms appear, victims face a rapid downward plunge, and death follows within one to 12 days.

No treatment for PAM currently exists; nor do we have a way to test water for the amoeba that causes it [source: CDC]. In two reported cases, doctors achieved some success using brain-swelling management and miltefosine, an investigational breast cancer and anti-leishmaniasis drug. But all other cases resulted in death, so the value of this and other drugs remains unclear [sources: CDC; Jacobson].

Our next contestant needs no introduction. Best known for killing 60 percent of Europe in the 14th century, it has helped to topple empires and drive social and technological change. Today, you can find it mainly lying low in small towns or villages in developing areas of Africa, Asia and South America, but it also enjoys the American West, where an average of seven cases per year popped up between 1900 and 2010 [source: CDC].

The Yersinia pestis bacteria cause three kinds of plague:

Bubonic sufferers quickly develop fever, headache, chills, weakness and enlarged, painful lymph nodes called buboes, where the plague bacteria multiply. Unless treated with the right antibiotics, this form of plague can spread throughout the body and develop into the other types. Septicemic patients add symptoms of abdominal pain, shock and possible internal bleeding, while pneumonic types exhibit a quickly worsening pneumonia that can result in respiratory failure and shock.

Thanks to antibiotics, plague's mortality rate has dropped from 66 to 11 percent. Untreated, septicemic and pneumonic plagues consistently kill their victims [sources: CDC; WHO].

The 2014 discovery of misplaced variola rekindled fears over the possible impact of a new smallpox outbreak among a population no longer vaccinated against it. The U.S. maintains a strategic stockpile of smallpox vaccine which, if administered within three to four days of exposure, can blunt infection and vastly decrease fatalities. Still, bioterror concerns persist, especially if the virus could be genetically altered or otherwise weaponized [sources: Aleccia; CDC].

Smallpox exists in two types. Variola major involves a more prevalent rash and steeper fever, while the rarer, gentler variola minor kills in only 1 percent of cases. Variola major is subdivided into ordinary (90-plus percent of cases), modified (a mild version that crops up among the previously vaccinated), and two rare and severe kinds called flat-type (aka malignant) and hemorrhagic. While ordinary variola major kills around 30 percent of the infected, flat and hemorrhagic are nearly always fatal [sources: Inglesby et al.; CDC].

The sick pass the disease via lengthy face-to-face interaction, direct contact with infected bodily fluids or contaminated objects or, rarely, when the virus goes airborne in an enclosed setting like a bus or plane. After seven to 17 days of noncontagious incubation, victims experience two to four days of flulike symptoms, followed by four highly contagious days of skin and mouth rashes. Over the following weeks, these spread across the body and develop raised bumps, which fill with thick fluid and develop distinctive indented centers. Finally, pustules show up, gradually scabbing over, falling off and leaving behind pitted scars. Smallpox remains contagious until the final scab drops off [source: CDC].

In 2014, the worst Ebola outbreak in history erupted in West Africa. Hardest hit were Guinea, Liberia and Sierra Leone, which together totaled 14,432 laboratory confirmed cases and 9,936 deaths as of March 7, 2015. The outbreak touched Nigeria (20), as well as Senegal (1), Mali (8), Spain (1), the U.K. (1) and the U.S. (4) [source: CDC].

For a disease that isn't that easily transmitted (it requires direct contact with infected blood or bodily fluids, although some in Africa may contract it by handling bush meat or infected bats), Ebola certainly inspires terror. Perhaps we fear the case fatality rate, which ranges from 25-90 percent, depending on the outbreak, and averages around 50 percent. Likely, the lack of any cure or quick, reliable test has compounded these fears. And then there's the horror of the disease's progression, which begins two to 21 days after exposure (eight to 10 days average) and includes unexplained hemorrhaging, fever, severe headache, muscle pain, weakness, diarrhea, vomiting and abdominal pain. And finally, there are those fears, stoked by frightful films and panicked pundits, that the disease will mutate into an airborne strain.

Whatever the reason, the fight against Ebola faces a further challenge: the central mystery over the virus's natural reservoir host. Experts suspect bats but, until they know for sure, we cannot predict how or where the virus will show up in humans and spawn an outbreak [source: CDC].

Unlike its close cousin Ebola, we know the reservoir host for Marburg hemorrhagic fever: the African fruit bat, Rousettus aegyptiacus. Scientists don't know for sure how bats transmit the RNA filovirus (a filament-like virus that encodes its genetics using RNA), but they suspect it happens in bat-infested mines and caves, where humans come in contact with bat feces or small droplets of bat fluids, like urine, suspended in the air. Once in humans, it spreads like Ebola, via direct contact with an infected person's bodily fluids. Marburg occurs mainly in Africa [source: CDC].

Following an incubation period of five to 10 days, the infected person will experience a sudden attack of fever, chills headache and muscle pain, followed roughly five days later by a maculopapular rash (a rash with raised, spotted lesions), most noticeable on the torso. Nausea, vomiting, chest pain, sore throat, abdominal pain and diarrhea often accompany the rash. Finally, symptoms will grow worse, branching into jaundice, inflamed pancreas, severe weight loss, delirium, shock, liver failure, massive hemorrhaging and multi-organ dysfunction [source: CDC].

Marburg HF, like Ebola, has no known cure, and fatality rates can range widely depending on the outbreak (23-90 percent) [sources: CDC; WHO]. Diagnosis also can prove difficult, as the virus's effects resemble other those of infectious diseases, such as malaria and typhoid fever. Some experimental treatments show positive results in nonhuman primates tests, but are not yet approved for human trials [source: CDC].

Some infectious diseases are as common as dirt and, under the right circumstances, about as harmless. It's only when they encounter a suitable environment -- or when humans weaponize them -- that they become a threat.

Take Bacillus anthracis, the rod-shaped bacteria found in soil all over the world. Its spores remain dormant until they enter the body, where they propagate and churn out harmful toxic byproducts. Domestic herds and wild animals can breathe in or consume spores in contaminated soil, plants or water. Although rare, humans can get it too, usually through encounters with infected animals or contaminated animal products.

Anthrax enters humans via four main routes:

Each kind has its own symptoms, which can take one day to two-plus months to appear [source: CDC]. Inhalation and gastrointestinal anthrax hit like a terrible flu, but gastrointestinal adds swollen glands and potentially bloody diarrhea or vomiting. The cutaneous kind involves blisters and ulcers, as does injection anthrax, which spreads faster and throws in fever and chills.

If not treated in time, anthrax can kill, but all types can be avoided and remedied using precautions and antibiotics. A vaccine exists, but is only meant for particularly at-risk adults. Some countries, including the U.S., have veterinary public health programs that vaccinate animals against anthrax.

Medical advances over the past half-century have dropped botulism's one-in-two kill rate down to 3-5 percent, but the nerve toxin produced by Clostridium botulinum bacteria (and sometimes strains of Clostridium butyricum and Clostridium baratii) is still one nasty customer.

Found naturally in soil and inside homes on floors, carpet and countertops, the bacteria usually makes news through a health emergency involving the foodborne variety. But there are four other types of botulism, including toxins arising from botulism-infected wounds, infant-swallowed bacteria, rare adult intestinal colonization or an overdose associated with therapeutic or cosmetic uses like Botox [source: LDHH]. In the U.S., around 145 botulism cases are reported annually, breaking down as 15 percent foodborne (sometimes linked to home canning), 65 percent infant and 20 percent wound (linked to injecting black-tar heroin). Because honey sometimes contains the bacteria, authorities recommend not feeding it to children under 12 months old [source: CDC].

Symptoms of foodborne botulism usually kick off around 18-36 hours after consumption, and present as blurred and/or double vision, drooping eyelids, slurred speech, dry mouth and difficulty swallowing. Infant botulism reveals itself through lethargy, weak crying, poor muscle tone, constipation and a tendency to not feed well. If untreated, both types result in muscle paralysis symptoms that spread to respiratory muscles, arms, legs and torso, ending in death from respiratory failure.

Botulism is hard to diagnose but treatable with an antitoxin. In foodborne cases, an enema or induced vomiting might also be in order, while wound bacteria might require a course of antibiotics. Even for survivors, though, respiratory failure can mean weeks or months spent on a respirator. Recovery can take weeks, and weakness can last for years afterward.

In late 2012, the National Park Service reported 10 confirmed cases of hantavirus among Yosemite National Park visitors, three of them fatal. Nothing hits home like a virus that averages a 38 percent mortality rate and occurs across western and central U.S. and Canada, as well as throughout Central and South America [source: CDC].

Hantavirus spreads through contact with the urine, droppings or saliva of rodents, most notably the deer mouse (Peromyscus maniculatus) that hosts North America's nasty Sin Nombre strain. Careful housecleaning and mouse-proofing can help, but disturbing areas frequented by mice can also stir up infectious agents and make them breathable.

In the Americas, hantavirus can develop into hantavirus pulmonary syndrome (HPS), and that's when the trouble really begins. Roughly one to five weeks after exposure, most victims experience fatigue, fever and muscle aches, and roughly half also experience headaches, dizziness, chills and abdominal ailments. About four to 10 days later, coughing and shortness of breath kick in as fluid fills the lungs. Lacking a specific treatment, doctors treat the symptoms, often resorting to intubation, and fatality chances rise to 36 percent [source: CDC].

In other parts of the world, hantavirus also can develop into hemorrhagic fever with renal syndrome (HFRS). Flulike symptoms typically set in one to two weeks following exposure, but can also include blurred vision, flushed face, rash, or inflamed or red eyes. Later, patients might develop low blood pressure, acute shock, vascular leakage and acute kidney failure. Again, doctors provide supportive care. Fatalities range from 1-15 percent [source: CDC].

In both HPS and HFRS, fatalities vary according to the strain of hantavirus involved.

A brain disease associated with dementia might seem like an odd topper to a list of public enemies like these, but of all the diseases listed here, only variant Creutzfeldt-Jakob disease, or vCJD, carries a guaranteed death sentence. Like classic CJD, vCJD causes a widespread brain protein to fold improperly, littering the brain with spongy deposits.

But unlike classic CJD, vCJD likely arises from the same agent as mad cow disease (bovine spongiform encephalopathy) and probably stems from consumption of infected cow products. Compared to CJD, vCJD affects much younger people on average and takes roughly three times as long to kill its host (14 months versus five). It also shows up differently in diagnostics and involves additional psychiatric and/or painful sensory symptoms.

Beyond dementia, within four months, vCJD sufferers also can exhibit poor coordination, involuntary muscle contractions (myoclonus) or jerks (chorea), or exaggerated reflex or tendon responses (hyperreflexia). There is no treatment for the disease, which inevitably leads to progressive brain degeneration and death.

In June 2014, the CDC reported more than 220 vCJD cases globally, most of them occurring in the United Kingdom (177 cases) and France (27 cases). Of the four confirmed cases in the United States, all appeared to have contracted the disease while abroad [source: CDC].