Warthaug
Contributor
Midnight Star:
Difficult question. Viruses and their host-specificity are much easier to address then bacterial specificity, so thanx for picking the easiest example
In order to productively infect a cell a virus must be able to do a few things. If any of these steps fail, the virus cannot successfully infect the cell:
1) Bind to the cell
2) Induce its internalization
3) Escape from the internalization step
4) Replicate
5) Assemble itself
6) Escape the cell
As it turns out, the way cells work isn't all that different between most ofrms of life - the way we humans replicate our DNA, make RNA, and internalize (endocytose) stuff is pretty much the same as the way all other animals do it (yes, even including sponges, anemones, and jellyfish), and for that matter, is pretty much the same as the way things like molds do it.
Because of this similarity, the factors which limit the animals a virus can infect tend to be quite specific to the virus.
For example, influenza virus can infect birds, humans and pigs. The reason for this cross-reactivity is that all three of these organisms express a protein on the surface of their cells which the virus can bind to (called ICAM-1 in humans). The virus cannot infect other organisms because they either express a form of ICAM-1 that influenza cannot bind, or don't express ICAM-1 at all.
Another example is HIV. HIV can actually enter the cells of a huge range of animals. But HIV itself can only replicate in humans, chimps, and a few other types of monkeys. This is because these species express a gene the virus needs for its replication. Other animals simply don't have this gene, so even though HIV can get into their cells, it cannot replicate itself.
And there are viruses which infect pretty much anything they touch. Adenoviruses are a good example of this. Although they are commonly thought of as a human virus, you put them on pretty much any cell (including insect cells, of all things) and they'll infect.
Bryan
