This may be Ian MacDonald's novella Tendeléo’s Story.
(On further research, it's more likely to be McDonald's Evolution's Shore series, consisting of the novels Chaga (a.k.a. Evolution's Shore) and Kirinya, as well as the novella Tendeléo’s Story. I haven't read the series yet (it's next on my ever-growing list) but it looks like the second book, Kirinya, takes place partly in India and partly in space (a mission to reach the source of the nano-spores). I've only glanced through it but there may also be an arousal-activated mutation as well.)
Tendeléo’s Story has the alien life form probe:
I was thirteen and just a woman when the Chaga came to my world and
destroyed it. That night I was at Grace Muthiga’s where she and I had
a homework club. It was an excuse to listen to the radio...We were
listening to trip hop. Suddenly the record started to go all phasey,
like the radio was tuning itself on and off the station. At first we
thought the disc was slipping or something...Then we heard the first
boom. It was far away and hollow and it rolled like thunder. Most
nights up in the Highlands we get thunder. We know very well what it
sounds like. This was something else. Boom! Again. Closer now...Most
High had the answer: ‘Sonic booms! There’s something up there!’ As he
said those words, we saw it. It was so slow. That was the amazing
thing about it...The announcer said that the biological package had
come down on the eastern slopes of the Nyandarua near to Tusha, a
small Kikuyu village. Tusha was a name we knew. Some of us had
relatives in Tusha.
And the mutations:
No, it was something, but nothing we could recognise. The photographs?
They only show the thing. They do not show how it happens. The houses,
the fields, the fields and the track, they run like fat in a pan. We
saw the soil itself melt and new things reach out of it like drowning
What kind of things?
We do not have the words to describe them. Things like you see in the
television programs about the reefs on the coast, only the size of
houses, and striped like zebras. Things like fists punching out of the
ground, reaching up to the sky and opening like fingers. Things like
fans, and springs, and balloons, and footballs.
The girl with mutations:
‘Ah, Mr. Giddens. You’re with Ms Bi, that’s right?’
‘That’s right, how is she?’
‘Well, she was brought in with multiple lacerations, upper body, left
side of face, left upper arm and shoulder…’
‘Oh Jesus God. And now?’
‘See for yourself.’
Ten walked down the corridor. If she had not been wearing a hospital
robe, I would have sworn she was unchanged from how I had left her
that morning...The weals were already fading from her face and hands.
A terrible prescience came over me, so strong and cold I almost threw
‘We want to keep her in for further tests, Mr. Giddings,’ the doctor
said. ‘As you can imagine, we’ve never seen anything quite like this
before...we’ve discovered the presence in her bloodstream of fullerene nanoprocessors.’
‘What are commonly know as Chaga spores.’
Dead bodies, although not exactly in a river:
Riverside had been rich once. I saw a tank up-ended in a swimming
pool, a tennis court strewn with swollen bodies in purple combats.
Chaga camouflage. Beyond the trees I saw fans of lilac land-coral.
The whole story takes place in sub-Saharan Africa, not India, and the ending doesn't really match.
The carpet of moss began to resemble a crackle-glazed Japanese bowl.
But they all focused on Ten. She was the source of the pattern. And
the Chaga-cover began to flow towards the lines of force. Shapes
appeared under the moving moss, like ribs under skin. They formed
grids and squares, slowly pushing up the Chaga-cover. I understood
what I was seeing. The lines of buried walls and buildings were being
exhumed. Molecule by molecule, centimetre by centimetre, Gichichi was
being drawn out of the soil.
Some of the mutated survivors gain the ability to control the alien nano-spores and use them to...? (the final outcome is left open, though it looks like the meek really will inherit the Earth)