What makes some viruses more deadly than others

[Iota]

Can someone explain this phenomenon in layman terms? I've read it's due to certain genes on the virus' RNA which create the proteins for that virus... but that's all I can find and that doesn't explain much other than the obvious; that some viruses are different from others.

 

 

[jp255]

There are a number of factors which can contribute to the virulence of a particular virus.

 

The type of cell which a virus can infect contributes to the symptoms an infected individual will develop. E.g HIV infects immune cells which has a CD4 receptor (a type of receptor the HIV virus uses to gain entry into a cell) so AIDs eventually develops, viruses which cause common cold symptoms infect cells in the nasal passage causing runny nose, fever etc. This is called cell tropism (the type of cell a virus has evolved to infect).

 

The severity of the immune response that an individual's body launches against a virus can vary depending on the virus, and the location at which the immune response occurs can result in more severe symptoms. An example of severe immune response is the Spanish flu pandemic which is believed to have caused a cytokine storm (immune overreaction) in young people, which is thought to be the reason why it was most lethal to young people (unusual for a flu virus, usually kills infants or old people). The references for this can be found on wikipedia, just search and read about spanish flu. I don't know why certain viruses or strains can cause stronger immune responses.

The location at which immune response occurs can affect the severity of the symptoms, eg rabies causing acute inflammation in the brain is very severe as opposed to common cold causing inflammation elsewhere.

 

Lastly, some viruses have virulence factors. In viral genomes there is information coding for the required machinery to carry out the basic steps on the virus replication cycle. Some viruses also have additional genes which act as virulence factors. These genes can bolster the ability of the virus to infect and replicate. The mechanism by which these genes act is often by inhibiting the our defence systems. For example, We have APOBEC genes which have a function such that they can interfere in the virus replication process (e.g by introducing a lethal amount of mutations into virus genome). HIV-1 has a virulence factor which can disrupt this defence system (which is APOBEC3G, part of the gene family APOBEC) by degrading the gene's proteins. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1933301/ ,there are also other functions of APOBEC3G.

Individuals infected with HIV usually die (eventually) primarily because the virus' mutation rate is at a level which allows it to evade the immune system, within one individual there are too many different mutants of HIV for the body to deal with. It is called a quasispecies.

 

You can search many of the terms I used there to provide you with a lot of further reading if you wish to know more.

Edited January 31, 2013 by jp255