Comment Evolution Control Committee said it best (Score 1) 321
I want to go home.
I want sex.
I want a cookie.
Some pieces of software are intended to be feature-complete replacements for competitors. Others approach the same or similar tasks using very different methods. This doesn't change the relative merits of the software and soda analogies to books. Software is a much better analogy than soft drinks.
The rareness of feature-complete replacements for books, except in certain non-fiction areas, detracts from the software analogy to be sure. It still makes a lot more sense than 'coke vs. pepsi.'
The bottom line is that Google Search doesn't work very well - at least, not anymore. While it previously supported search expansions which could be taken advantage of by skilled searchers, it's since been focused on quick, lowest-common-denominator responses to the most common questions. As a result, searching for slightly abstract notions is virtually impossible, and some searches which should be straightforward also fail.
One example of a simple failure: "fireworks today" or "fireworks today san francisco" returned nothing after I chanced to see fireworks the other night. Using the date ("fireworks san francisco may 21 2014"), the only relevant result was a set of Coast Guard and DHS documents describing safety precautions for the event (Giants game). Of course, fireworks games are well publicized outside of interntal government safety documents.
A more abstract example: try to design a search for articles about names which are or have become insults, such as "Dick."
A mother who had been infected for a long period of time might have multiple strains due to accumulated mutation. About 25% of the time the fetus becomes infected, and in about half of these cases only one of the mother's strains will appear in the infant, which is usually assumed to mean that only a single virus of that strain was able to penetrate the in utero defenses (which are quite good, considering 0 viruses get through ~75% of the time).
To be clear, it's not that all the infections across individuals begin with the same strain, it's that each individual infection (frequently) does.
As dreamchaser said, HIV isn't actually all that easy to transmit, so in many cases it is only a single virus which begins the infection. In the other cases (heterogeneous initial infection) there was probably a unusually high viral dose.
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