A virus that appears to be a new strain of swine flu separately infected a boy in Indiana and a girl in Pennsylvania, the Centers for Disease Control and Prevention said.

Both children experiended flu-like symptoms including fever, coughs, shortness of breath, diarrhea and a sore throats. The fact that they both contracted the illness without coming into contact with one another has the CDC investigating how the illness spreads. If people can be infected by other people, rather than by pigs, the disease more difficult to contain.

We have been able to detect a novel flu virus, CDC spokesman Tom Skinner told WebMD. It is an H3N2 swine flu virus that has picked up a gene from 2009 H1N1. We are investigating whether human-to-human spread is occurring.

The new virus also poses a threat because it incorporates part of H1N1 but also boasts a unique structure, making it harder to guard against.

Pandemic viruses get started when they reassort and they emerge as a new virus. That is why we have to keep close watch on new influenza viruses as they emerge, Skinner said. They are constantly changing, and that is why we have to have really good surveillance systems in place to detect them when they do emerge.

The Indiana boy has been treated and recovered, while the Pennsylvania girl has not received treatment and is still sick. The girl came into contact with pigs at a state fair and a child care worker who watched the boy reported that he had had contact with pigs.

The World Health Organization estimates about 18,450 people died between when H1N1 originated in Mexico and the United States in March of 2009 and August of 2010.