Strigoi society and biology are intimately interwoven; you can't really talk about one without the other. So, first, the biology of the "vampire":
The strigoi "contagion" that leads to the profound biological changes of "vampirism" is passed through the strigoi's saliva, which also contains an anticoagulant. The vector of the contagion is not known (i.e., I haven't bothered to think it up because it doesn't matter a twit in the story), but a single bite on the neck is usually not enough to induce the change in the bitten. In the old days, before the Starveling Years, strigoi fed from mortals entirely via bites...which, sooner or later, led to the bitten mortals becoming strigoi, which led to a population explosion, depletion of prey humans...and all manner of bad times. In modern society, blood is harvested from humans via sanitary medical methods, and the "bite" is reserved only for very, very special occasions, including investitures in which a mortal is welcomed into postmortal society belowground.
After infection, the mortal's body begins to change slowly. Marked physical changes don't appear very quickly: the individual usually becomes very lethargic, pale, and suffers from constant thirst. Light aversion becomes prominent after a few days, especially when the first physical changes do become apparent (often first in the eyes, which begin to grow more and more dark-adapted and intolerant of multispectral light), but full photophobia doesn't usually set in until the skin begins to change after about a week. The skin begins to grow greyish, somewhat scaly, and very dry--almost powdery in some cases--and begins to blister if exposed to multispectral light (though weak, low-frequency [reddish] light is still okay). Hair begins to fall out at this point, too. The convert's dietary needs begin to change at this point: he or she becomes incapable of digesting solid food by the end of the second or, at most, third week. He still needs a lot of water, but soon begins to crave human blood, which is all the strigoi will need to stay alive from that point forth--provided, of course, that the blood comes from healthy, well-fed individual mortals. Once the diet begins to change, the teeth begin to fall out. Once all adult teeth have come out, the characteristic "strigoi needles" grow in at the position of the two top frontal incisors. These teeth are long and very sharp and often overhang the lower lip. Think Nosferatu here. Fingernails and toenails begin to grow very rapidly and thicken; "wild" strigoi in the old days often sported wicked claws for snagging prey, but almost all civilian strigoi--those not involved in military applications--clip their nails down like most mortals.
The most profound changes happen very rapidly at around the end of the fifth week. At this point, photophobia is extreme and any strigoi exposed to daylight will simply sizzle away to nothing. The diet now consists entirely of human blood, the skin has become pale grey and completely hairless, the eyes squinted and almost all pupil, and the lower jaw has begun to shrink. Up to this point, most converts still look mostly human--if somewhat goofy with their pointy little rat teeth and big bald heads. But then the real change kicks in. Once the strigoi has gone belowground, their ears begin to grow, eventually to become large, mobile, and very bat-like. The acuity of their hearing increases a thousand-fold and it's usually at this point that they begin to experiment with echolocation, which is somewhat instinctual, but taught to all newborn strigoi to help them get better at it fast. At this point, no matter how old the mortal was at the point of conversion, the strigoi is now youthful, strong, fast, and quickwitted. Physical aging does not technically stop during conversion, but it slowed down to a snail's pace. Strigoi are ultimately mortal...but no one really knows how long a strigoi can actually live. The oldest are at least two thousand years old. Most die by violence or accident long before then, though. Nonetheless, the strigoi long ago learned to carefully maintain their numbers to avoid the population crash inevitable with a population whose birthrate greatly exceeds its deathrate.
Physical changes do occur in strigoi after conversion. After about two or three hundred years, unless exposed to carefully controlled light, the eyes will eventually become vestigial and disappear. The extreme eldest strigoi--like the Emperor, the Pontiff of the Church, and some others, who never leave their subterranean homes--are all blind, their eyes grown over with skin; they get by entirely by echolocation. Strength and agility fade by the thousand-year mark, and by that point, the lower jaw has also degraded to little more than a useless hinge to support the lower lip, giving the older strigoi an almost mosquito-like appearance. In some cases, fingers and toes grow longer and longer for some unknown reason until the strigoi can no longer functionally pick up anything in their hands and are entirely dependent on mortal, ghul, or younger strigoi servants.
Strigoi hearing is thousands of times better than a mortals: the frequency-perception range of their ears is faaaaaaar beyond mortals. Hence, strigoi music is full of high and low tones imperceptible to mortals...making a strigoi "symphony" sound as choppy and weird to the mortal ear as an Autechre track run through a band-pass filter and a noise filter. Strigoi speech generally features vowels and frequencies inaudible to mortals, hence: the Strigoi High Speech. Almost all strigoi, though, can still form the sounds and words necessary to speak various mortal tongues, the Lower Speeches. Of particular interest, as far as sounds go, is the strigoi "screech," a unique howl of absolutely ear-splitting ferocity that the strigoi can use to either disorient mortals, alert other strigoi of danger, or sometimes use as a severely nasty epithet in conversation. Kind of like screaming "FUCK!" through a 10,000-watt amp.
Strigoi are not pretty. They are, ultimately, just overgrown haemovores with many adaptations to living in almost total darkness. They also just happen to live VERY long lives, need little maintenance, and are at the absolute top of the food chain.
Next time...what all this has to do with strigoi society.