A septic truck carrying a full load of literal human waste had a mechanical failure. Human waste is one of the largest bio hazards that are common in our society. Unlike blood, we excrete waste daily, and this waste can carry all kinds of bacteria and viruses. In fact, you can contract AIDS and hepatitis C from poop.
With the mechanical failure of the tank, all the sewage bursts out onto the road where there are many cars. It quickly fills up the area of the road, and this stuff will likely go on to contaminate the local water supply.