Copyright suit and jurisdiction

We are an Indian firm and have developed a CMS application to manage websites. Application runs on UK servers. We are having clients from all over the world and their websites are hosted on the same UK server. We have received a civil suit summon for copyright infringement of CMS software by a United States District Court for the Central District of New York. Plaintiff's software firm is claiming that we are having a client from New York whose website(website for one of the New York's school) is managed using our CMS and so we are making business in New York and using infringing material for commercial gain in New York. Hence, jurisdiction is district of New York. I am a Indian citizen having firm registered in India operated from India only. We sale our software online which is hosted on UK servers. Our New York's client site is hosted on UK server. Our CMS software is running on same server. 1. How can we come under jurisdiction of United States District Court? 2. What should be our reply to a such summon?