Half Life (II) - S1.E17

The NIH team teams up with the officers of the 55th Precinct in New York City to try to figure out what is the source of an outbreak of Marburg, a deadly disease with no known cure. They also try to locate patient zero, a murder suspect on the run, who is helping to spread the disease throughout New York City.

The crossover starts on Third Watch S06E16 In the Family Way (I)