autism: n. 自闭症zi4 bi4 zheng4, zheng4 means illness
autistic: a. 患自闭症的 huan4 zi4 bi4 zheng4 de, huan4 means "get (the disease)", or you could replace huan4 with 得de2 or 有you3, totally the same meaning, it's juest 患huan4 is more formal.
"de", when without any tune, is sort of an adjective suffix in Chinese, but NOT in all cases, you can see it as a suffix here, for your easier understanding.
for instance, an autistic child, in Chinese would be:
an autistic child:
yi2 ge4 huan4 zi4 bi4 zheng4 de er2 tong2, 一个患自闭症的儿童 or
yi2 ge4 de2 zi4 bi4 zheng4 de er2 tong2, 一个得自闭症的儿童 or
yi2 ge4 you3 zi4 bi4 zheng4 de er2 tong2 一个有自闭症的儿童
get it?