A man was arrested in Texas for sexually assaulting two girls, a 14-year-old and a 2-year-old, and in the process infecting them with HIV and other sexually transmitted diseases. Houston police said the 33-year-old man is a registered sex offender.

The man, Darren Wilson, was charged with two counts of sexual assault of a child. The children were his ex-girlfriend's teenage daughter and his neice.

An investigation began in November 2013 when the younger child was taken to Texas Medical Center in Houston with an unusual growth on her genitals. She was infected with herpes and chlamydia, and doctors performed reconstructive surgery on the growth.

The older child was also diagnosed with the same STD, and doctors alerted investigators that both of the girls showed clear signs of sexual abuse.

Four family members who had regular contact with the girl were then questioned by officials, and all were asked to be tested for any sexually transmitted diseases. Only Wilson's came back positive for HIV, and he had recently been treated for chlamydia, according to the police.

The teenager also reportedly told a social services worker in September that she had been having a sexual relationship with the suspect and was pregnant.

According to Houston police investigators, the younger child's mother and father live out of state and are not involved in the girl's life.

The man had been sentenced to four years in prison in 2004 for sexually assaulting another 14-year-old child, according to the Texas Department of Public Safety's sex offender registry.

Wilson was taken into custody Thursday and is being held without bail at the Harris County jail. He was expected to make a court appearance Friday, according to officials.

Neither the victims' families nor Wilson's attorney have made any public comments regarding the charges.