May a man treat a woman or a woman treat a man?

Sahih al-Bukhari · Book 76 · Hadith 5679

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، عَنْ خَالِدِ بْنِ ذَكْوَانَ، عَنْ رُبَيِّعَ بِنْتِ مُعَوِّذٍ ابْنِ عَفْرَاءَ، قَالَتْ كُنَّا نَغْزُو مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم نَسْقِي الْقَوْمَ، وَنَخْدُمُهُمْ، وَنَرُدُّ الْقَتْلَى وَالْجَرْحَى إِلَى الْمَدِينَةِ‏.‏

Narrated Rubai bint Mu`adh bin Afra: We used to go for Military expeditions along with Allah's Messenger (ﷺ) and provide the people with water, serve them and bring the dead and the wounded back to Medina.