Using that strategy, the minute marauders have flourished and evolved in step with their hosts. By one estimate , at least , different viruses can infect mammals alone, and even this massive number may be on the low side. This viral army can cause symptoms as mild as a cough or as deadly as internal bleeding. Some viruses may even cause the runaway cellular growth that is the root of cancer, as is thought to be the case with human papillomavirus and cervical cancer.
Inside their cellular hosts, viruses can create an enormous number of copies and spread the infection to other cells. For example, if you get the flu, your body will be riddled with some hundred trillion viruses in just a few days —more than 10, times the number of people on Earth.
How viruses spread from person to person depends on the type. Many hitch a ride in the mist of droplets that flies from your mouth every time you cough or sneeze. A variety of factors can influence how fast these airborne viruses can spread. Flu, for one, seems to survive longer in cool, dry environments , which may be the source of its common winter spread.
But in tropical regions, high humidity seems to help the flu jump from person to person. Other viruses spread most easily through contact with other bodily fluids. For example, Ebola virus spreads from contact with infected blood, feces, or vomit.
Unlike many other viruses, scientists think Ebola cannot spread through the air after people with the virus cough or sneeze. Still other viruses travel through an intermediary, like a mosquito, which then infects people by biting them.
One example of these so-called mosquito-born diseases is dengue, which causes a potentially deadly flu-like infection. The risk of dengue has risen in recent years, currently threatening roughly half of the global population, according to the World Health Organization. Other notorious mosquito-born diseases include Zika, Chikungunya, and West Nile.
Some scientists believe that viruses were fairly late to the evolutionary game, forming as remnants from cells that had somehow lost the ability to replicate. But other experts suggest that viruses could predate Earth's most ancient critters. The giant viruses have a surprising amount of independence compared to their tiny counterparts, so could have provided the building blocks of the diversity of life we know today. By one hypothesis , the first complex life originated from a cell enveloping a virus or, alternatively, a failed viral takeover.
Either way, the virus became a permanent cellular resident, forming the first nucleus. However, scientists can't even agree on whether viruses are truly alive. To be considered a living thing, an organism must be able to grow, reproduce, and generate energy on its own.
Some researchers also suggest that living things must be able to respond to stimuli and evolve over time. Viruses can't generate their own energy, and though they can reproduce and even evolve with the assistance of a host, those functions are impossible for one of the tiny entities out on its own. When the virus reproduces faster than the immune system can control it, it begins to destroy cells and harm the body. Viruses are also the smallest germ, making them generally the easiest to contract—they're so tiny they can spread through the air in a cough or a sneeze.
Some viruses also are spread by mosquitoes or through bodily fluid. Since each virus is very different, no one drug exists to attack whichever virus is in your body.
Vaccines give preemptive protection from certain viruses by training the body's immune system to recognize and attack a specific virus. Common forms: Bacteria cause food poisoning, strep throat and urinary tract infections , as well as infections such as tuberculosis.
Bacteria are bigger and more complex than viruses, though they can still spread through the air. A bacterium is a single cell, and it can live and reproduce almost anywhere on its own: in soil, in water and in our bodies. For the most part, we live peacefully with bacteria—the colonies in our guts are helpful to us and strengthen our immune system. But like viruses, bacteria can also harm us by replicating quickly in our bodies, killing cells.
Some bacteria also produce toxins which can kill cells and cause an outsized, damaging immune reaction. Broad-spectrum antibiotics were developed to kill bacteria in our bodies and in the food supply by inhibiting their growth.
But bacteria are extremely adaptive and can quickly evolve to evade antibiotics. Bacteria share their antibiotic-resistant genes with each other, meaning more strains generate resistance to the drugs we use.
Common forms: Fungi are responsible for causing conditions such as yeast infections , valley fever and meningitis. Fungi are more complicated organisms than viruses and bacteria—they are "eukaryotes," which means they have cells. Of the three pathogens, fungi are most similar to animals in their structure.
There are two main types of fungi: environmental, which are yeast and mold that often live in soil and don't generally cause infection in most healthy people; and commensals, which live on and in us and generally don't hurt us.
Bacteria are giants when compared to viruses. The smallest bacteria are about 0. This makes most viruses submicroscopic , unable to be seen in an ordinary light microscope. They are typically studied with an electron microscope. Their mode of infection is different. Because of their distinct biochemistry, it should come as no surprise that bacteria and viruses differ in how they cause infection. Viruses infect a host cell and then multiply by the thousands, leaving the host cell and infecting other cells of the body.
A viral infection will therefore be systemic , spreading throughout the body. Pathogenic bacteria have a more varied operation and will often infect when the right opportunity arises, so called opportunistic infection. The infection caused by pathogenic bacteria is usually confined to a part of the body, described as a localized infection. These infections may be caused by the bacteria themselves or by toxins endotoxins they produce.
Examples of bacterial disease include pneumonia , tuberculosis , tetanus , and food poisoning. Viruses can infect bacteria. Bacteria are not immune to viral hijackers which are known as bacteriophages —viruses that infect bacteria. Image by Venngage Infographic Maker.
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