The CHICAGO star - who is fearful Asia could be "lost" to the epidemic - is travelling to the subcontinent with the American leader this week.
He says, "The president of the US coming there in a world where he's seemingly obsessed with terrorism, clearly obsessed with it, and talking about HIV/AIDS in the same breath, in the same paragraph - is extremely important."
"India is a country that I care about deeply. We're talking about a population in India that is close to a billion people. If this crisis hits them to the degree it's expected to, we've lost Asia."
The 56-year-old Buddhist is also keen to use his global fame to rally supporters for his cause: "What I felt that I could bring to this that maybe someone else couldn't, was the ability to talk to people at a high level in government, in business and in my own community of actors."