Having just had a very unpleasant experience in the Cordillera Huayhuash using the trekking agency 'Huascaran' in Huaraz, which the Lonely Planet guidebook says 'gets repeatedly good reviews' (the main reason why I chose them) I would like to show that there is another side to the agency and advise to you think twice before using them.
Everything was going fine on the trek until about midnight on the second night when my stomach started to churn. There then followed some of the worst diarrhoea of my life, with added vomiting (only the time I had giardiasis was worse). This continued throughout the following day leaving me too weak to trek with the group, and so another agency member had to accompany me as I walked at a snail's pace with regular lengthy 'toilet stops'. I eventually had to hire a horse to take me to camp as I may not have made it by nightfall otherwise. I had only drunk bottled water by this point, plus the boiled water for the tea (and no other group members were ill) so it must have been the food that was responsible. One of the other group members told me later that he didn't eat the chicken we had for lunch as he noticed his piece wasn't cooked through properly, so this was the likely culprit. On the morning of the fourth day I was in no state to continue the trek and was within walking distance of a village with transport links to Huaraz, and so left the trail with a staff member from the agency, but in some ways the worst was yet to come.
The owner of the agency, Paulino, flatly refused to accept any responsibility for what had happened to me or to give even the slightest refund as I was the only one to fall ill. He insisted it was impossible that some of the chicken was ok and some not as it all came from the same chicken, not seeming to understand that cooking time was the relevant factor with bigger pieces needing longer than smaller ones. Worryingly, he'd also never heard of Salmonella. He called me a liar when I mentioned the group member who saw their chicken was undercooked, and claimed that food poisoning can only take effect several days after eating contaminated food (not true, it depends on the amount of bacteria present) and so it must have been food I ate before the trek which made me ill. Though possible in theory, as I'd been backpacking in South America and Africa for a year by this point, eating in local restaurants without illness, it's strange that the place I ate the night before the trek should be the first one to make me ill, some 55 hours later, coincidentally on the same day some chicken served by the agency was found to be undercooked. Personally I doubt he even believed everything he said, it seemed more of a justification to keep all of my money.
No doubt some of you will have used Huascaran and had no problems arise, and so will not have had the misfortune to see the more unpleasant side of Paulino's character reveal itself. It may be claimed that food poisoning could happen with any trekking agency, (though I would argue otherwise) but the agency's response to it is certainly under their control and reveals their attitude towards their clients and their degree of professionalism. Though of course cheap, disreputable companies will behave similarly, as one of the more expensive agencies I found for trekking Huayhuash I expected much better from them. Writing this, I find myself on antibiotics, having had a trek I was greatly looking forward to ruined by an nasty illness due to bad practice by the agency employees, with a large amount of money lost in the process. I would advise you not to take the risk that you will end up in similar circumstances and to give your money to a more deserving character than the shady Paulino and an agency with a better understanding of food safety. (Additionally, the inflatable mattresses all had punctures, the zips on the tents were nearly all broken and I was cold in the aged sleeping bag on the second night despite wearing my warmest clothes, including coat and woolly hat. I imagine the agency gets good reviews due to the friendly guides and regular snacks, but they'll be found in other agencies too.)