A virus’s life

Virus is a Latin word meaning poison. Viruses do not eat, breathe, move or reproduce on their own. So then, what kind of life is it? None, viruses are not technically alive. A virus is a little box that contains instructions to make other little boxes just like them. Each virus has its own shape: the measles virus has a spherical shape, that of ‘flu is a twenty-sided solid, that of polio and the coronavirus look like hairy spheres, the viruses that infect bacteria are shaped like spaceships with little legs for landing. Different structures but all tiny: the largest virus has a diameter of 300 nanometers, so a row of 3,000 of these ‘giants’ would be 1 millimetre long. The smallest one measures just 20 nanometres, and to make a millimetre you would have to put 50,000 of them one after the other.

Viruses cannot replicate on their own and to do so, they must enter the cells of living organisms. Every organism has its own viruses: viruses that attack basil plants do not harm us, those that harm snails do not make human beings ill. A virus recognises the right cells and attacks them. How does it do it? Small spike proteins protrude from the surface of the cells of all living things. Viruses have them too, and if they fit into those of the cell, like a key in a lock, the virus is absorbed. The cell digests the virus and releases the instructions inside it. It’s the cell that builds the pieces of the new viruses. The next step for all newborn viruses is to leave the organism all together and infect others. In fact, a virus is programmed to reproduce in abundance and spread everywhere and by any means. For example, some tickle our noses making us sneeze or make us cough so they can be transported elsewhere with the mucus. This is what all these viruses do: influenza, Ebola, cold, smallpox, meningitis, polio and COVID-19, the latest coronavirus. Other viruses pass from one organism to another twhen organic fluids, such as blood, saliva and sperm are exchanged: this is how HIV, herpes and hepatitis C viruses spread. Then there are viruses that are carried by insects, such as the yellow fever virus that uses mosquitoes. Other viruses are spread by food: they are the ones that cause hepatitis A and E.

Our body’s immune system is the police headquarters that defends us at all times from virus attack. Lymphocytes are the police force produced by our immune system. They are special cells that patrol the whole body through blood and lymph and if they find viruses they produce antibodies. Antibodies are proteins that attach themselves to the virus to make it harmless. They’re like handcuffs that stop a criminal before he does any wrong. Antibodies prevent the virus’s spike proteins from becoming attached to the cell’s proteins. Lymphocytes are smart, but sometimes viruses are faster and can make us ill.

Vaccines are the deadly weapons that defend us against viruses. Let’s see how they work. A vaccine contains only little bits of the virus. These little pieces are not dangerous but, like mug shots, they show the lymphocytes what the criminals look like. Thanks to vaccines, the lymphocytes already know what the viruses look like, so they can equip themselves with the appropriate antibodies before the viruses show up. If viruses of the same type, but natural and active, later attack a vaccinated organism, the immune system is already prepared to deal with them.

The term vaccine clearly derives from vacca (the Latin word for cow). But why does a medicine have a name deriving from the Latin word for cow? Smallpox is a terrible viral disease that has killed millions of people around the world. Edward Jenner was an English country doctor who around 1770 observed that milkers and cattle rearers did not get smallpox. He discovered that the smallpox virus that attacks cattle can be transmitted to people but is not lethal, so he sensed that contact with sick beasts could make people immune. He therefore tried injecting the infected serum of a bovine pustule into the body of a young patient: the boy did not catch smallpox. After more than 20 years of study, the country doctor perfected the invention that has saved, and continues to save, billions of lives. The vaccine still bears this name today in memory of Jenner’s studies. Thanks to the vaccines, smallpox no longer exists. The last case of poliomyelitis in Italy dates back to 1982. It is worth remembering this when we have doubts about the effectiveness of vaccines.

Each virus makes its home in a specific organism. There are viruses that only infect tomatoes, others that only affect horses, and others that only affect chickens or just people. However, some viruses manage to leave its usual host to occupy another. A disease that passes from animals to humans is called zoonosis. Almost 60% of the pathogens that cause human illnesses are zoonoses. This is what led to this coronavirus pandemic. The Ebola virus, for example, passed from monkeys to humans. HIV, the AIDS virus, was also found in African monkeys before it killed 30 million people. Sometimes, before infecting people, some viruses are carried by an intermediate animal: avian influenza viruses were initially detected in chickens, then in pigs and finally in humans. The Hendra virus is hosted by Australian bats but first it infected horses, then their breeders. Zoonoses are difficult to eradicate. In fact, the smallpox and polio viruses that only affected humans, thanks to vaccines, have practically disappeared all over the planet. However, it is impossible to know in advance in which animal the virus of the next zoonosis is hiding. A new one can always develop… unexpectedly.

There is no living being immune to virus attack, not even bacteria. Spillover is the natural phenomenon whereby a virus that affects animals, changes, mutates and becomes able to attack human beings. To understand how this works, let’s go back to the mechanism whereby a virus and a cell recognise each other. If the virus’s spike proteins fits into the proteins that protrude from the cell, like a key in a lock, then the infection occurs. But every cell has its own lock. So viruses that have the right key for infecting snail cells do nott affect our cells, and vice versa. However, viruses reproduce quickly and in large numbers, so errors in the production of copies are always possible. It may be that the key of an animal virus changes by chance, mutates and becomes compatible with the lock of human cells. This happens more frequently in coronaviruses, which can acquire the ability to pass from animals to humans more easily. Covid-19 was probably a virus that normally attacks bats. Spillover occurs more easily where there is prolonged contact between humans and animals, for example, where there are large farms or where wild animals are part of the human diet. The more contact between animal and man is prolonged, the more likely it is that a virus will accidentally change and become dangerous for humans.

Most new viruses originate in tropical forests. These, in fact, are the richest and most complex environments in the world. Millions of different species live in these environments: animals, plants, fungi and micro-organisms, some of them as yet unknown, and viruses and parasites that are equally unknown. As long as the forests are intact and healthy, the viruses remain in the creatures that host them while they wander through the tall trees. Where wildlife is killed and trees are cut down to make roads, fields, timber, pastures and then mines and cities, the natural home is taken away from viruses and other pests. The more we enter an environment to destroy it, the more likely it is that humans will come into contact with unknown viruses and pests. A parasite disturbed in its daily life and evicted from its regular host has two possibilities: it can disappear or, unfortunately for us, find a new home.

By Andrea Bellati