DALLAS — Texas health officials today stepped up their efforts to fight against any possible outbreak of Ebola by increasing the number of people they are monitoring to as many as 100, including most who were secondary contacts of the first diagnosed case of the disease in the United States.

The sharp increase from the 12 to 18 people announced on Wednesday comes as the city continues to deal with the fallout from the case of Thomas Eric Duncan, a Liberian who arrived in the United States last month, began developing symptoms and then tested positive for Ebola.

“These additional people had some sort of contact with those initial 12 to 18 individuals. Some are classmates of students who had contact with Mr. Duncan. They’re all being closely monitored and watched,” Erikka Neroes, public information officer at Dallas County Health and Human Services, said.