This isnt one of those crazy questions that everyone wants to know i dont think. Just a simple question about viruses that ive had no luck finding the answer to.
Why dont viruses attack vectors? For instance Rodents can carry many diseases that dont affect them, why doesn’t the virus try to multiply in the rodents body killing the creature?
Obviously it wouldnt be good for the virus to attack the vector for the viruses sake but how does the virus know the difference between human and rodent cells?. I know the virus doesnt make decisions about who or what it attacks, its just kinda coded to do so but still it intrigues me, as does many things in life.
Viruses most commonly do attack their vectors. Rabies makes almost any mammal very ill. Ducks get moderately ill from influenza while pigs become just as ill as humans and occasionally die. Cowpox is a minor ailment in both humans and animals. The list of viral zoonoses is quite large. In many instances the disease has dramatically different effects in the animals and human hosts. SARS for example is a mild disease of civets but moderately dangerous in humans.
There are species specific viruses out there that can infect people but not rats, and these are occasionally transmitted by rats. The reason rodents can carry such viruses that don’t affect them at all is because the viruses travel externally. Viral particles become trapped in the skin mouth of the rodent and are passes onto human in this way. We humans carry viruses that affect animals without ever becoming ill ourselves in exactly the same manner. Foot and mouth is a classic example of such external transmission of a virus. People infect animals by transporting the disease on clothing and skin without ever needing to become infected themselves.
Viruses that are species specific are unable to affect non-host animals for any number of reasons.
The primary mechanism is the ability to lock onto host cell. In order to infect a virus needs to chemically bond to the host cell. It does this by a reaction between the virus coat protein and proteins or sugars on the surface of the host cell. In distantly related species these proteins and sugars are chemically very different and the virus can’t latch on, and so infection is prevented.
The hosts immune system also prevents cross species infections in many cases. In order to infect new cells the virus needs to float free in the body. In most viruses the surface proteins mimic the host proteins to some degree. This fools the immune system long enough to allow the virus to find some new hosts. If a rat virus were to enter a human the surface proteins would not be a close match for human protein and the virus would be recognised and destroyed instantly.
Temperature is another major consideration. Rats have a core temperature several degrees hotter than a human. As a result many rat viruses that enter the human body can’t operate efficiently at the colder temperatures, while human viruses entering a rat are slowly cooked.
There are a range of other factors that also prevent cross species infection, but I’m too lazy to go into them all.
From the virus’ perspective, a very low level or asymptomatic infection is usually best, because if it kills the host, it needs to find a new host to remain reproduce in. The major exception to this is manners of death that encourage spreading to lots of new hosts, such as diarrheal diseases that get virus-loaded feces everywhere, infecting tons of new hosts.