Condoms made from sheep intestines, labeled "lambskin", are also available. Although they are generally effective as a contraceptive by blocking sperm, it is presumed that they are likely less effective than latex in preventing the transmission of agents that cause STDs, because of pores in the material.[75] This is based on the idea that intestines, by their nature, are porous, permeable membranes, and while sperm are too large to pass through the pores, viruses—such as HIV, herpes, and genital warts—are small enough to pass through.[73] However, there are to date no clinical data confirming or denying this theory. Some believe that lambskin condoms provide a more "natural" sensation, and they lack the allergens that are inherent to latex, but because of their lesser protection against infection, other hypoallergenic materials such as polyurethane are recommended for latex-allergic users and/or partners. Lambskin condoms are also significantly more expensive than other types and as slaughter by-products they are also not vegetarian.