Like many viruses, influenza (the flu) benignly lives and evolves in other animals. When people contact infected animals (such as ducks, which commonly harbor influenza), novel and potent viruses can cross over into the human population.

It has long been hard for people to accept that they can be hurt, or even killed, by something they cannot see. But this is exactly what viruses do and have always done; they evade our body’s immune system, hijack our cells to create more viruses, and have consequently caused epidemics throughout human history. More recently, we’ve recognized they can also cause cancers, as well as other changes to our very genomes that we have yet to fully understand the implications of.

Smallpox: Possibly the oldest viral disease on record is smallpox, with accounts dating to 1100 B.C. in China and India. It made its way to Europe around 500 A.D., and then infamously followed the Europeans to the Americas in the early 1500s, destroying Native American populations. Because it is eradicated today, it can be hard to imagine just how difficult life with this disease was; of the people who caught it, up to forty percent died, mainly through damage to their heart, kidneys, or brain. People who survived could still have injured kidneys, disfiguring scars, or blindness (which it was the leading cause of).

Luckily, in the late 1700s, an observant British scientist named Edward Jenner noticed that dairymaids previously infected with “cowpox” (which caused red blisters on cow udders, and is similar to a mild case of smallpox in people) had immunity against smallpox. Using scabs from infected cow udders, Jenner infected several hundred thousand people with cowpox, effectively vaccinating them against smallpox. But even with a vaccine, like many viruses smallpox remained a significant health problem for years; in 1967, ten to fifteen million people were infected, and over two million died. The World Health Organization took action and created an intensive, $300 million program to finally put a stop to the disease, and it worked; it was officially eradicated in 1979.

Measles: Measles has caused epidemics in Europe and Asia for about two thousand years. Like smallpox, measles made its way to the Americas with early Europeans and wrecked havoc on the previously unexposed Native Americans. Given how contagious it is, its destructive ability is not so surprising; measles is possibly the most contagious virus in the world, even more than the common cold, as a person can catch it just by breathing air in a room where an infected person was two hours earlier. And it can kill about ten percent of the people it infects, if they face malnutrition and inadequate health care, with weeks of high fevers, rashes, severe diarrhea, dehydration, and other infections. Trying to survive measles was simply a part of life for much of human history. In 1963, a vaccine was developed and the disease mostly vanished in developed countries. Like smallpox, measles too could be completely eradicated from the human population if a global, stringent vaccination policy were carried out for several years because, unlike many diseases, measles doesn’t live in any other animals we know of; we’re its only home. While the number of lives measles claims each year is declining, killing about 873,000 people in 1999 down to 164,000 in 2008, it remains a significant cause of mortality for children in many developing countries.

Herpes: Like measles, herpes (caused by the herpes simplex virus 1 and 2) has also caused known epidemics for about two thousand years. When the virus multiplies, it creates blistering lesions, which spread across the infected area and gave the virus its name; herpes is Greek for “to creep.” While simplex 1 targets the skin and mucous on the lips and mouth, creating “cold sores,” simplex 2 attacks these kinds of tissues in the genitals. It’s been long known that the virus spreads through the contact of these sores; the Roman emperor Tiberius Julius Caesar Augustus banned kissing possibly in efforts to prevent the rampant spread of herpes throughout Rome.

While the body’s immune system eventually fights off herpes, it isn’t gone for good. A persistent viral infection, herpes simply retreats when “beaten,” waiting to return and fight another day. Where does herpes retreat to? It actually hides in the body’s nerve cells, sometimes for years before returning to cause lesions. While there’s no cure yet, there are treatments to control its severity and several vaccines in trials.

The virus that causes chickenpox (the varicella-zoster virus) belongs to the same family as herpes simplex, and retreats in a similar manner. After causing chickenpox in children, the virus retreats to nerve cells (in the vertebrae), but can reemerge years or decades later and cause the painful rash condition of shingles.

The Epstein-Barr virus, which causes mononucleosis (“mono”) and may infect over ninety five percent of all people at some point, also belongs to the herpes family. Like chickenpox, it can make a repeat performance later in life, particularly in the form of cancer; it’s been associated with causing Burkitt’s lymphoma, nasopharyngeal carcinoma, and several other cancers.

Influenza: While it is hard to track the historic spread of influenza (also known as the flu) as its symptoms resemble a respiratory disease, it’s thought that, along with smallpox and measles, it too made its way from Europe to the Americas in the early 1500s. Altogether, the three viruses killed as much as ninety-five percent of some Native American tribes.

But the worst influenza epidemic was yet to come; in 1918, half of the world came down with influenza. Within months, between forty and one hundred million people (about five percent of the world’s population), mostly healthy young adults, died from the “Spanish flu.” It was the worst pandemic in modern history. It’s also why scientists and physicians were terrified in 2009; the influenza strain of 1918 was also H1N1. Luckily, the 2009 H1N1 influenza strain, the “swine flu,” did not prove to be as virulent as the 1918 strain.

So why is the flu so effective, and why is there a different flu vaccine every year? The protein “coat” that surrounds the influenza virus’ is what our immune system uses to recognize and attack the virus, but the virus changes this coat all the time, making it hard for our immune system to identify. Part of why it changes all the time is because it doesn’t just live in humans; influenza often benignly lives in the digestive tract of pigs, ducks, and other waterfowl. There it swaps genes with other influenza strains, and occasionally a very potent virus is created this way. Consequently, where people are often in contact with these animals new influenza strains often find their way into the human population, such as in many Asian countries.

Rabies: While people in the U.S. today may be remotely familiar with rabies from the 1957 Walt Disney Classic “Old Yeller,” the disease was a serious threat to everyday life until the early 1900s. After being bitten by an infected animal, a person often developed rabies, which included a fever followed by depression, restlessness, frothing, unquenchable thirst, and certain death, if untreated. Luckily, the brilliant French chemist Louis Pasteur developed a vaccine (using dried fluids from infected animals’ spinal cords, where the virus resides). Unlike most vaccines, the rabies vaccine can be used after a person has been in contact with an infectious source because the virus can take a week to over a year to infect a person after exposure. In 1885, Pasteur first successfully used his rabies vaccine on a 9-year-old boy who was attacked by a rabid dog. Within the next couple years, Pasteur’s vaccine saved about 2,500 lives, and has saved many more since. While rabies is rare in the U.S., it is still common in developing countries, killing around forty to seventy thousand people annually.

Hepatitis: Around the same time that Pasteur was curing rabies, people were inadvertently spreading hepatitis while reusing immunization needles to vaccinate against other viruses. Hepatitis had yet to be identified, and people did not know that they were developing yellowish skin (jaundice) due to viral liver damage. This symptom is actually caused by three different viruses, hepatitis A, B, and C. Hepatitis A (HPA) is the mildest, spreading orally from bad food or water in unclean living environments and causing nausea, vomiting, and jaundice for a month or two, but then allowing for complete recovery.

Hepatitis B (HPB) is much more serious. HPB is very infectious, spreads through sexual contact or body fluids, and is often deadly. Worldwide, about three hundred million people (five percent) have a chronic infection, mostly in Southeast Asia and tropical Africa, but this also includes 1.25 million Americans. Around one million people die from HPB annually, due to liver disease (cirrhosis) and the five million liver cancer cases it causes. Luckily, a vaccine was developed in the 1970s, making it the first cancer vaccine.

It took researchers a bit longer to identify hepatitis C (HPC), the most deadly of the hepatitis viruses in the U.S. Isolated in 1989, HPC was found to also cause liver disease and cancer. In the U.S., it infects nearly four million people, occurring more frequently than any other blood-borne infection here, and killed ten thousand in 2001. While many drugs can be used to treat it, and it’s become the leading cause of liver transplants in the U.S., unfortunately, because HPC mutates so quickly, a successful vaccine has yet to be developed.

Poliovirus: While HPC remains a significant problem for many in the U.S., polio (caused by the poliovirus) has mostly been eradicated from this country. 1916 witnessed the first U.S. polio epidemic, and it remained a significant fear for four decades. Like hepatitis A, polio spreads through contaminated, swallowed substances, such as from swimming in a public swimming pool. While most infections go unnoticed, as the virus stays in the intestines, sometimes it enters the nervous system and proceeds to kill neurons, often resulting in paralysis. After realizing this in the late 1940s, American medical researchers Dr. Jonas Salk and Dr. Albert Sabin developed different vaccines. While it is now rare in the U.S., polio is still a major concern in South Asia and sub-Saharan Africa, where, in 2000, nearly four thousand were afflicted with pain and paralysis, and hundreds of thousands more were infected. Efforts to perform a global eradication are underway, but it has been difficult to implement vaccination programs in countries with social and political upheaval.

Ebola: Ebola has caused many of the most recent viral epidemics. Discovered near the Ebola river valley in Zaire in 1976, Ebola killed over ninety percent of people in over fifty villages there in a fast, gruesome manner, with hemorrhagic fevers. The virus is transmitted to people from animals (most likely infected carcasses), and then spread between people through the contact of contaminated bodily fluids. Although there have been multiple epidemics since 1976, because the virus is actually fragile without a host, and kills a person within a week, the epidemics have remained fairly localized. While there is no vaccine available or standard treatment, several vaccines are in trials.

HIV: Human immunodeficiency virus (HIV-1), the virus that causes acquired immunodeficiency syndrome (AIDS), is currently a pandemic that has infected nearly forty million people, and has killed over twenty five million people since it was recognized in 1981. HIV-1 attacks the host’s immune system, causing a person to die from opportunistic infections. Although HIV-1 is from a chimpanzee virus (the simian immunodeficiency virus [SIV]), SIV does not cause a disease in the chimpanzee, but it does in other non-human primates. While anti-viral treatments are available, vaccines are not, although there have been promising stem cell findings based on individuals who are somewhat resistant to HIV-1 infection. HIV-1 is an ongoing, significant problem, with nearly three million new cases reported in 2007 and two million deaths. For now, perhaps the best weapon against HIV-1 infection is awareness.

Cancer: Although we’ve known that viruses can cause cancer since 1911 (with the discovery of the Rous sarcoma virus, a chicken virus), the idea has only recently been widely accepted by the scientific community. Because many viruses reproduce by inserting their genes into our genomes, they can cause cancer; these insertions can (usually randomly) disrupt important genes we have for preventing cancer, or insert their own cancer-promoting genes. In addition to the Epstein Barr virus and hepatitis mentioned above, another prominent example is the human papillomavirus (HPV), which causes cervical cancer and kills a quarter million people globally (four thousand in the U.S.). Luckily, a vaccine against HPV, and consequently against cervical cancer, has been developed.

Because viruses have this habit of inserting their genes into our genomes, it may not be surprising to learn that our genome is actually eight percent viral DNA, specifically belonging to bornaviruses. These RNA viruses first started their invasion over forty million years ago. Today, it’s unclear what the implications are for our genome; some of the viral genes surprisingly appear to be beneficial, while others may not be (and may even cause schizophrenia).

Either way, scientists have been using viruses in research for decades, such as in studying how different, introduced genes behave in other organisms, or, more recently, in promising new gene therapies to treat genetic diseases. It appears as though the tables may be starting to turn; instead of viruses using us to their advantage, as they’ve done for millions of years, we’re learning how to apply their unique abilities to further our knowledge and ourselves.

For more on viruses and their origins, see Luis P. Villarreal’s book “Viruses and the Evolution of Life,” the book “The Biology of Viruses” by Bruce A. Voyles, Barry E. Zimmerman and David J. Zimmerman’s book “Microbes and Diseases That Threaten Humanity,” and Wikipedia’s article on “Virus.”

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at


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