I've never used anything stronger than caffeine and alcohol myself, so this is all from reading and some limited experiences with people who have used drugs.
For starters, injection is not the only way to take heroin. It's also smoked, snorted and sniffed. But you are right - heroin is rarely the first drug of choice for anyone. It's not glamorous, and it has a "bad image". Thankfully that reduces recruitment. However once people have tried it, the "natural inclination to not jab yourself with a needle" is quickly overcome with habituation.
A substantial gateway to heroin usage is abuse of other opoids such as prescription painkillers. Ironically many of the people who make the switch does so because they find heroin is cheaper and more easily accessible than commonly prescribed opoids that are often safer (not least because they're of predictable strength), available in pill form etc.
I haven't looked up any significant studies at source, but here's an article from the National Institute on Drug Abuse that claims that 23% of heroin users become dependent [1], and an article [2] that covers some work on the subject of heroin addiction and casual use, though, that includes the following quote:
"In the early 1970s, researcher Lee N. Robins led a study commissioned by the Department of Defense that followed tens of thousands of Vietnam War veterans as they returned to the U.S. Use of narcotics and heroin was rampant among soldiers stationed in Southeast Asia, with as many 20% showing signs of addiction. Yet during the first year back, “only 5% of those who had been addicted in Vietnam were addicted in the U.S.” and “at three years, only 12% of those addicted in Vietnam had been addicted at any time in the three years since return, and for those readdicted, the addiction had usually been very brief.” It wasn’t for lack of access to junk, either: half of the returning addicts said they’d tried heroin at least once since arriving back home.
As my Reason colleague Jacob Sullum has documented, such take-it-or-leave-it findings are common in drug research. In his 2004 book Saying Yes and other places, he’s detailed work in which researchers find a surprising range among heroin users, including a study that concluded, “It seems possible for young people from a number of different backgrounds, family patterns and educational abilities to use heroin occasionally without becoming addicted.”
The 23% number above is the highest estimate of percentage of addicts amongst heroin users I've ever seen, and as far as I understand it reflects the lifetime risk of becoming addicted at some time. As noted in the quote from the Time article, many addicts go on to stay free of abuse for years at a time subsequent to becoming addicted.
But to be clear, whether at 23% or 10% or 2%, heroin addiction is not something to take lightly. It's a nasty drug. However, it's not nearly as nasty as it is generally portrayed, and many of the biggest problems for heroin users, and addicts, are a result of it's status as an illegal drug rather than due to the drug itself. Secondly, as noted above, many of them come to heroin due to lack of access to other, safer, drugs - a typical cascading problems with drug abuse.
My "closest encounter" with heroin for my own part was working with an addict - a well functioning one for most of the time I knew him. As for many heroin addicts, he first ran into problems when prices spiked overnight due to supply problems: Suddenly it was hard for him to feed his habit from his normal income, and he started spending a disproportionate amount of time on the phone, seemed jittery and nervous, and eventually a couple of k disappeared from the till at the store he co-owned, and he disappeared for a couple of days before checking himself into rehab.
That's the first we knew of his addiction.
As long as he had a steady supply, he was a nice, hard-working family man with a loving wife and son that knew little to nothing about his addiction. That's not to say that the heroin was not a problem for him - he was certainly an addict, and it certainly would affect his health over time. But he was far from the typical image of heroin addicts you get from anti-drug propaganda. He opened my eyes to considering that the addicts you see sleeping rough etc. are not the full set of heroin addicts, or even most.
As with alcohol use, heroin addicts also manage to function in many or most cases, and as with alcohol use, most users are not addicts - not even according to the National Institute on Drug Abuse with their 23% number. The biggest challenge to keeping things together for the guy in question was finding a safe and stable supply for whenever he felt unable to stay clean. The biggest threat for him, then, was government drug enforcement supposedly there to keep us safe from drugs, that prevented him from getting a guaranteed clean supply at a cost that could let him focus on fixing his life and keeping his job and family, and that would have substantially reduced his health risks.
I used to be against all legalisation, but I've come to see almost all anti-drug legislation as downright immoral. Even when it comes to heroin. It doesn't mean I think it ought to be available at the grocery store next to the milk, but I do think trying to prevent access has a vastly higher human cost than allowing carefully regulated sale would.
[1] http://www.drugabuse.gov/publications/drugfacts/heroin
[2] http://ideas.time.com/2013/11/21/trey-radel-scandal-whats-so...