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Why do viruses infect people?
Viruses like COVID-19 are obligate pathogens. That means they depend on hosts to survive. Specifically, they need to infect host cells. Cells are the building blocks that make up the body. Viruses can find ways to get into those cells and use them as factories to make more copies of themselves.
Every virus is different. Some need more support from their hosts than others. However, all of them are unable to survive without host cells to infect. Viruses infect humans, animals, and even plants! Without a host, a virus cannot survive for long. But viruses are not able to enter the body without facing some key challenges first.
What barriers do our bodies use to protect us from viruses?
Your body has a few levels of protection against pathogens like viruses. The first obstacle is physical barriers like skin and mucous membranes. These act like armor for our bodies. Sweat, oil, and mucus can be sticky situations for viruses to escape in order to get to your cells.
But what happens when a virus gets past physical barriers? The next challenge is to pass by the complement system. This system gets its name because it complements, or helps, the cells of your immune system in fighting a virus as soon as it arrives in your body. This system immediately targets pathogens for destruction, but it is not always successful.
Some viruses arrive at the next barrier – your cells. Cells are incredibly intelligent. They have built in mechanisms to grow, divide, process nutrients, and even fight pathogens. Some cells have a specific job of sensing pathogens. Their sensors are called Pattern Recognition Receptors (PRRs). When viruses are nearby, PRRs can detect them and sort them out from familiar, self, molecules.
How do the PRRs know that they have encountered a pathogen? Viruses leave hints called Pathogen Associated Molecular Patterns (PAMPs). Once those PAMPs come into contact with PRRs, your cells know that something non-self has entered the body. Then, they take action.
When PRRs receive a PAMP signal, they cause a cascade of cell responses by passing on the message to other cellular helpers. It becomes a domino effect. One outcome of this is that the cell sends signals called cytokines and chemokines out into the body. Those signals warn your immune cells that something dangerous is happening. This gives your body the opportunity to respond to the infection immediately by sending immune cells to the site of infection. It’s as if your PRRs have called an ambulance! Once the immune cells get to the infected area, they can start fighting the virus.
Another result of the PRR domino effect is cell death. There are many different types of PRRs. When some of them are activated, your infected cells may respond by dying.
Why do infected cells choose to die?
Cell death sounds scary, but it’s actually a really helpful mechanism that the body uses to protect us from disease. Think of it this way – if one leaf on a tree is not very healthy, it might fall off. But the rest of the tree might be strong and happy! Similarly, if one cell is infected, it may not be able to function in a healthy way with the rest of your body. Instead, that cell will be told by PRRs to die. This protects the remaining cells from damage and infection.
However, some cells survive infection. This can be a bad thing! If an infected cell does not die, the virus that has infected the cell can survive and cause more damage. Scientists have found that some types of viruses do not kill cells. Instead, they interrupt cell death signaling and cause the cells to stay alive. It’s as if the virus adds a block in front of one of the dominos, stopping the rest of them from falling down. This allows viruses to keep replicating and spreading to other cells.
What is our final defense against viruses?
If the virus has made it this far, it can be bad news for your body. You can get sick and pass the sickness to other people. The good news is we have one last line of defense – adaptive immunity. When viruses infect your cells, at least some of them get broken down. Infected cells then present the broken pieces to your immune cells as evidence. This way, your immune cells know what to look for next time that virus comes into your body. This can help them to act faster and fight back before you get sick!
Some viruses can be especially tricky. For those, we need extra help from medicine. Vaccines can teach our bodies how to fight without ever seeing the real live virus. They introduce your body to things that look like the virus, called antigens. The antigens teach your body what the virus looks like in case it’s ever infected
From your skin to your adaptive immune system, your body challenges any pathogen that might need a host. With all of these barriers to infection and with a little help from medicine, we can conquer any dangerous virus!
Pathogen – disease-causing microorganisms
Virus – a type of pathogen that can only replicate in a host
Obligate Pathogen – a pathogen that needs a host to survive and replicate
Cells – building blocks of the body
Mucous Membranes – surfaces of the body that are covered in mucus
Complement System – a system that fights pathogens as soon as they enter the body
PRR – Pattern Recognition Receptor – a sensor that detects pathogens
PAMP – Pathogen Associated Molecular Patterns – “hints” left by pathogens that can activate PRRs
Cytokine/Chemokine – signals sent by cells to warn the body of infection
Adaptive Immunity – a system that helps the body to remember what dangerous pathogens look like