Viruses are perhaps, one of the earliest and simplest life forms in the universe. Size alone doesn’t explain why viruses survived the asteroid strike that wiped out the dinosaurs. Oh! And No! Viruses aren’t immortal but survive and flourish especially in favorable environmental conditions. Therefore viral infections persist.
Going by the definition of ‘living or non-living’ things, viruses vaguely fit into a clear cut classification. Consequently, this dents the precision of grouping and characterization of life or its purpose.
Viral infections remain the greatest threat to human existence. The 2019-20 coronavirus pandemic, for one, has re-emphasized the detestation of viruses. Why then do viruses exist?
Do viral infections exist? To kill?
Well, a consensus of the scientific community believes that viruses are merely alive by virtue of their inability to reproduce independently.
As a matter of fact, viruses that kill are actually quite a few compared to the total number of viruses in reality. Carl Zimmer of National Geography gave a hint as he wrote, ‘every time you get sick with the flu, for example, every infected cell in your airway produces about 10,000 new viruses.’
Bestowing to popular belief, the total number of flu viruses in your body can rise to 100 trillion within a few days of viral infection. This invariably means there are over 10,000 times more viruses than people in the universe.
So, ‘if there are that many viruses in a single person, how much more are there in the universe? To answer this, Carl concluded that there are over 10 million times more viruses than there are stars in the universe.
For most viral infections, the symptoms experienced are usually as a result of our immune system trying to eliminate the virus. This in turn renders our bodies miserable, so that we stay inactive and devote available resources to repelling the attack.
The death of a viable host invariably means no more replication and the virus most likely goes extinct.
How do pathogens infect the hosts?
Different mechanisms adopted include:
- Viral infection through droplet — the virus can easily be passed along when an infected person sneezes on another e.g SARS-CoV-2
- Viral infection through airborne transmission — this happens to be one of the most effective means of transmission for most viruses. It occurs through exhalation and inhalation. This is how Tuberculosis is transmitted.
- Viral infection through vector transmission — viruses are so small, they can slip into other vessels as tiny as bacteria or get picked up by a slightly larger carrier such as a mosquito and transmitted to a new host. Yellow fever is transmitted this way.
- Viral infection through waterborne transmission — this mode of transmission occurs when a host (mostly animals) infects a water source with droplets present in bodily fluids such as saliva, blood, sweats, or feces. These pathogens are accidentally taken up (e.g., in drinking water) by a new host who would further spread it. This is how Cholera is transmitted.
- Viral infection during dormancy — technically, viruses require a host cell to replicate, without which some viruses remain dormant. This means it is able to live outside a host for long periods of time until they come in contact with a new host. An example is the variola virus causing Smallpox.
Eventually, viruses that fail to meet explore these mechanisms would never infect a new host and would go extinct.
Why do viral infections kill?
To fully understand why some viruses are so viciously virulent, we assume a microscopic standpoint. To humans, pathogens such as viruses and bacteria are villains, assailants of our bodies. Ironically, on the other hand, they just seek to evolve and thrive. Hence, viruses exist for the same reason people, bacteria, fish, and plants do.
Now, let’s put this into perspective. Consider shelter, a unique ecological challenge faced by any thriving specie, yes, including microbes. Appropriate, comfortable habitats can be few and alarmingly far between. Imagine yourself -as a virus- in a comfortable, natural habitat — a human host.
You have secured an apartment (by infecting some cells )and managed to reproduce, but your Landlord ( host’s immune system) vows to make life unbearable for you by turning up the heat.
Without being told, this environment is no longer hospitable. As a logical being, the next line of action would be for your descendants to relocate to a friendlier habitat (probably another human). For this, you require a means of transportation ( legs, wings, fins, or any of the usual means of locomotion) without which it’ll be a herculean task for your descendants to reach a new host on their own.
Strikingly, however, natural selection has been established such that these viruses are equipped with a number of sneaky ways to make giant strides to a new host.
What happens to a virus when it kills the host?
Infection and transmission is a matter of life and death for pathogens. For viruses, this has been perceived as an evolutionary trade-off between virulence and transmission.
Nonetheless, natural selection balances this trade-off, selecting viruses virulent enough to produce numerous offspring. The aim is to infect new hosts without incapacitating the current host. The mode of transmission also affects its vicious virulence. For instance, the novel coronavirus can hide beneath asymptomatic and mild cases to cause community spread.
On this premise, some biologists hypothesize that the reason for this trade-off puts into perspective why viruses that cause sexually-transmitted infections tend to linger.
A highly virulent virus exploits its host to produce more offsprings than most within a period of time. The resultant effect would be that this virus causes more damage to the host. Excessive exploitation would incapacitate the host and hinder interacting with new, potential hosts. Thus, the transmission is greatly limited.
An exception however is seen in Cholera. There is rapid deterioration. Its casualties are soon immobilized and incapacitated by the infection. Nonetheless, cholera victims are cared for by others who carry away their waste, clean their soiled clothes, and, in the process, transmit the bacterium to a water supply where it can be ingested by new hosts.
What host’s factor favors the virulence of pathogens
A huge factor is the level of immunity of the host population. Other factors include; the distribution of the hosts in a herd (community) and underlying health conditions of the host. The novel coronavirus takes a greater toll on the senior citizens whose immune systems are impacted by terminal and lifestyle diseases. This suggests how we can tweak pathogen evolution towards making them less virulent.
Reducing transmission rates (e.g. through physical distancing, good hand hygiene, self-isolation, and uncompromised surveillance) may halt transmission or favor less virulent forms. The ultimate goal is to eradicate hyper-virulent strains that soon kill or immobilize their hosts.
Viruses exist as an integral part of nature. As they evolve and thrive, a few pretty much disrupt any host from microscopic bacterium to intellectual beings like humans. As a specie, we grow, fight back, and intellectually evolve and adapt.
Just as there are friendly bacteria that exist in our intestines and help us digest food, humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli (E. coli).
A few viruses are incorporated into human DNA, becoming an indispensable part of our genes. New evidence has proved that this lies behind some key features of mammalian pregnancies.
Vaccination is also available to boost herd immunity against particularly harmful viral infections. Essentially, not all viruses are deadly but commonplace viral infections can be dealt with a person with compromised immunity.
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